The Prevalence of Low Back Pain in Adults: A Methodological Review of the Literature.

Key Words: Low back pain, Prevalence.

One definition of low back pain (LBP) is any back pain between the ribs and the top of the leg, from any cause.[1] Low back pain is an important public health problem in all industrialized nations.[2,3] Although most people appear to recover quickly from an episode of LBP, disability resulting from back pain is more common than any other cause of activity limitation in adults aged less than 45 years and second only to arthritis in people aged 45 to 65 years.[1]

Prevalence is the number of people in a defined population who have a specified disease or condition at a point in time.[4] Prevalence is usually measured by surveying a particular population containing individuals with and without the condition of interest.[5] Thus, prevalence equals number of people with a health problem at a point in time divided by the total defined population alive at this point in time. Prevalence rates are usually reported as percentages. Prevalence is the result of many factors: (1) the periodic number of new cases, (2) the immigration and emigration of people with disease, and (3) the duration of illness.[4] Because LBP is a condition that often resolves completely, individuals can be counted as having prevalent cases if pain reoccurs. This is different from chronic conditions that do not resolve (eg, rheumatoid arthritis).

Point prevalence is measured at a single point in time (ie, the number of people reporting LBP on the day of a survey).[6] Period prevalence is measured over a specified time period, usually 1 year (ie, those people who report having had LBP in the past 12 months). In contrast, incidence refers to the number of new cases occurring during a period of time among a group initially free of the disorder.[4] The cumulative incidence or lifetime incidence is the total number of people who have or have had the condition during their lifetime. Deyo and Tsui-Wu[7] also refer to this as the cumulative lifetime prevalence. Thus, it appears that there is confusion in the literature with respect to the use of the terms "lifetime incidence" and "lifetime prevalence." Figure 1 illustrates 10 cases of LBP in a hypothetical population of 20 individuals from 1993 to 1998. Calculations of period prevalence, point prevalence, annual incidence, and cumulative incidence are provided as examples to clarify definitions.

[Figure 1 ILLUSTRATION OMITTED]

Numerous authors have estimated the prevalence of LBP in various populations. Unfortunately, methodological differences among studies and lack of methodological rigor make it difficult to draw accurate conclusions. Methodological issues relevant to studies that estimate LBP prevalence have been identified.[8,9] Table 1 lists these issues. In summary, it is important to have (1) a random sample that is representative of the target population, (2) a measure of outcome that yields valid and reliable results, (3) a definition of LBP, and (4) a report of response rates and a comparison of respondents and nonrespondents so that the generalizability of the sample can be determined. Recently, formalized guidelines for the critical appraisal of prevalence literature have been developed by Loney et al.[10] Table 2 describes the criteria needed to critically assess articles determining prevalence. The reader is referred to Loney et al[10] for further elaboration and clarification of the criteria. Articles that have sound methodological rigor will provide more precise estimates of prevalence, with minimal bias.

Table 1.
Methodological Items to Consider in Population Surveys
of Low Back Pain (LBP)

Item                                 Study

Random sample representative         Leboeuf-Yde and
  of target population                 Lauritsen,[8] Volinn[9]
Reasons for no response described    Leboeuf-Yde and Lauritsen[8]
Response rate reported               Leboeuf-Yde and
                                       Lauritsen,[8] Volinn[9]
Comparison of respondents
  and nonrespondents given           Volinn[9]
Primary LBP data used                Leboeuf-Yde and Lauritsen[8]
Same mode of data
  collection for all subjects        Leboeuf-Yde and Lauritsen[8]
Point prevalence
  estimates provided                 Volinn[9]
Outcome measure(s)                   Leboeuf-Yde and
  validated or tested                  Lauritsen,[8] Volinn[9]
Question regarding                   Leboeuf-Yde and
  LBP specified                        Lauritsen,[8] Volinn[9]
Precise definition of LBP given      Leboeuf-Yde and Lauritsen[8]
Recall period clearly stated         Leboeuf-Yde and Lauritsen[8]
Type of survey administration
  specified
  (ie, phone/personal
  interview, assessments)            Volinn[9]

Table 2.

Guidelines for Critically Appraising Studies of the Prevalence of Health Problems[10]

1. Are the results of the study valid?

Is the study design appropriate for the research question?

* Ideally a cross-sectional survey should be used Are the study subjects obtained appropriately?

* A random sample, (stratified if appropriate) of the target population needs to be identified

Is the sampling frame appropriate?

* Must be the best possible (ie, census data) to minimize bias Is the response rate adequate?

* The greater the number not available for measurement, the less valid the prevalence estimate

Are objective and suitable criteria used for measurement of the issue or concern?

* Outcomes need to be valid and reliable, definitions and details of the survey questions need to be provided

Is the concern or outcome measured in an unbiased fashion?

* Assessors/interviewers should be trained and blinded, if possible

2. What are the results?

Are the estimates of prevalence or incidence given in detail?

* 95% confidence intervals should be provided Is the sample size adequate?

* A large sample size will produce a smaller error rate and smaller confidence intervals Are the results given by subgroup, if appropriate?

3. What is the applicability of the results?

Are the study, subjects and the setting described in detail and similar to those of interest to you?

* Study sample should be described in enough detail to determine the generalizability of the results to your population

Will the results lead directly to a health care or policy, decision?

Accurate determination of the prevalence of LBP is important to physical therapists and other health care professionals for a number of reasons. First, once the true prevalence rate is known, the societal impact of LBP in terms of cost and disability can be assessed accurately. Second, knowledge of the prevalence rate would aid in the organization and prioritization of information for health service planning and development. Currently, numerous practitioners are diagnosing and providing varying types of care for patients with LBP.[7] Some interventions being used have not been shown to be effective.[11] Third, with accurate prevalence estimates, increased funding for research aimed at improving the diagnosis and treatment of this condition may be possible. Finally, physical therapy educators might think of expanding course curricula to provide more study time for the most prevalent conditions seen by the physical therapy profession.[12] If LBP is more prevalent than other conditions seen by physical therapists, then the management of people with back pain may need to be re-evaluated.

The purposes of this review are (1) to apply the aforementioned methodological guidelines to the literature on community prevalence of LBP and assign an overall methodological score, (2) to compare studies deter mined to be methodologically acceptable in an attempt to draw conclusions about the prevalence of LBP in the world population, (3) to estimate of the point prevalence of LBP in North America, and (4) to make suggestions for improving the methodological quality of these types of studies.

Methods

Search Strategy

Some authors[13] have suggested that several overlapping search strategies should be used to ensure that as many as possible of the available articles are included in a literature review. A search of MEDLINE, Cumulative Index to Nursing and Allied Health (CINAHL), and Science Citation CD-ROM systems from 1981 to 1998 was conducted for this review. The Medical Subject Headings (MESH) headings "low back pain" (focused to epidemiology) and "prevalence" were used. The search was limited to studies printed in English and to the study of adults (ages 19-64 years). Only community-based prevalence studies were used for this review. Studies of groups in the community (eg, health care workers, industrial workers, military) were excluded.

The search strategy for this review was as follows:

Step 1. The reference lists of articles identified by MEDLINE (1981-present) were searched for other relevant publications. We refer to this process as a hand search. MEDLINE citations and hand-searched articles were grouped together and called the MEDLINE search strategy.

Step 2. CINAHL CD-ROM (1981-present) was searched. Selected articles were hand searched for relevant publications. Relevant citations and hand-searched articles were grouped together and called the CINAHL search strategy.

Step 3. Science Citation CD-ROM (1981-present) was searched for appropriate articles.

Estimating the Completeness of the Search

Capture-recapture methods are derived from the technique used in ecological studies in which animals are captured, marked, released, and recaptured for population estimates.[14] Recently, these methods have been applied in epidemiology to estimate the degree of overlap between 2 or more search strategies. Simple formulas are used to obtain an estimate of the total size of the population, also known as the horizon,[14] or, in our search of the literature, the total number of articles available. This procedure enables a researcher to estimate the number of publications that were not identified by the search strategy in order to evaluate the completeness of a systematic literature search.

The first step of our process was to define the search strategy and the steps for obtaining articles for the review. We did this to ensure the reproducibility of the search, which will allow other researchers to judge the validity of our strategy. Spoor et al[14] used articles in a journal found by MEDLINE versus an independent hand search of the same journal. The search strategy for our review is defined above. In this case, the hand-search component was combined with the associated CD-ROM search because articles obtained by hand searching were found from references obtained by the CD-ROM search. Next, we constructed a contingency table to show the number of articles identified by each search strategy and the amount of overlap between strategies. Only 2 search strategies (MEDLINE and CINAHL) were appropriate, because no articles were found in Science Citation. Thus, a 2 X 2 table was constructed (Tab. 3). The contingency table shows the extent of overlap in the number of articles found and not found by the MEDLINE and CINAHL search strategies. In this table, the cell of "not found" by both search strategies (X) is unknown, and it is this cell that will ultimately be estimated.

[TABULAR DATA 3 NOT REPRODUCIBLE IN ASCII]

It is beyond the scope of this article to provide the derivation of the formulas used in the capture-recapture calculation, and the reader is referred to Bishop et al.[15] Using the formulas provided, the total size of the population of articles (N) can be estimated.[15] Once N is known, X can be calculated using simple algebra. Finally, 95% confidence intervals can be obtained for the point estimate. All calculations are provided in Table 3.

Critical Appraisal and Scoring

The primary author appraised each article using a worksheet that summarizes the criteria outlined by Loney et al[10] (see Appendix for a sample of the worksheet used). A scoring system was developed to rate the quality of the studies reviewed (Tab. 4). We agreed that each item would be weighted equally because we did not believe that one item was more important than the other items. Each item was given a score of 10 points, with the maximum score being 90 points. The weighting of items equally has support in the literature. Streiner and Norman[16] explained that unequal weighting of items in a scale consisting of relatively homogeneous items contributes relatively little, except added complexity. Dixon et al[17] used equal weighting in their rating scales for the critical appraisal of articles on clinical agreement, diagnosis, causation, therapy, prognosis, and overview.

Table 4.
Methodological Scoring System Used to Rate Studies Reviewed

Item                                              Score

1. Random sample                                10 points
2. Unbiased sampling frame (ie, census data)    10 points
3. Adequate sample size                         10 points
4. Outcomes valid and reliable                  10 points
5. Adequate response rate                       10 points
6. Point prevalence estimates provided          10 points
7. Confidence intervals provided                10 points
8. Definition and duration
    of low back pain given                      10 points
9. Study refusers described                     10 points

Maximum score                                   90 points

A sample size of 300 or more has been suggested as adequate for population surveys examining dementia,[18] The prevalence of LBP, however, is likely higher than that of dementia. According to Kachigan,[19] the sample size required to estimate a proportion with a specified degree of precision can be determined using simple formulas. Proportions nearer 50% require larger sample sizes than smaller proportions.[19] Using a conservative sample size estimation for proportions (prevalence=50%), with an error in estimate of less than 3% at the 95% confidence level, the calculated sample size is 1,067.[19] Thus, we considered a sample size of 1,000 to be adequate for the purposes of this review. A response rate in population surveys of two thirds to three quarters has been suggested to be generalizable to the population samples.[20] Thus, a response rate of 70% was chosen as acceptable. A priori, a total methodological score of 70 points was deemed acceptable. This cutoff was chosen because we believed that these studies would be methodologically sound to enable generalization and to provide a basis for discussion and conclusions.

Specification of a Comparison Measure

Point prevalence was chosen as the primary epidemiologic comparison measure (ie, subjects' report of LBP at the time of the survey). Although other measures of prevalence are provided in many studies (ie, period and lifetime prevalence), point prevalence has the advantage of not being based on recollection. "Memory decay" is a phenomenon associated with increasingly forgetting events (eg, past episodes of LBP) with the passage of time.[9] The longer time has passed, the more likely a person is to forget the event. This would tend to cause an underestimate of the reporting of LBP event, thus lowering the prevalence rate. Recollection bias was supported in a study by Carey et al.[21] In contrast, the phenomenon of "forward telescoping" (tendency to recollect events as occurring more recently then they actually did) tends to have the opposite effect on the prevalence rate.[9] Forward telescoping tends to increase the reporting of LBP in a specified time period, thus overestimating the prevalence rate. Carey et al[21] stated that these recall biases may offset each other. The use of point prevalence, however, is supposed to eliminate such assumptions and thus was used as the main comparison in our review of the literature, although other prevalence rates will be provided and compared.

Results

Eighteen studies were reviewed from the years 1981 to 1998. Twelve of these studies were conducted in Europe, 5 studies were conducted in North America, and 1 study was conducted in China. All studies dealt with the prevalence of LBP in adults. The studies yielded different types of prevalence estimates (ie, point, period, lifetime) and included different subgroups of the community. The researchers used different durations of LBP in their estimates. Generally, the definition for LBP was consistent. Some authors, however, did not define LBP. Many researchers used a body diagram for clarification of the definition of LBP.

Completeness of Search Strategy

Using the capture-mark-recapture technique, the estimate of the total population size of the available articles was 18 (95% confidence interval=16,20), rounded to the nearest whole number (Tab. 3). The number of articles missed by this search strategy was estimated to be 0. Thus, the overall search strategy can be considered complete.

Methodological Quality

Table 5 presents the results of the critical appraisal of articles reviewed and the overall methodological quality scores calculated.[7,22-38] Three studies[22,26,29] were given scores of 70 points or more and were considered of high quality. Studies given scores of 45 points or more were considered to be of moderate quality and were selected for further comparison to determine trends in methodology and prevalence rates. Van Tulder et al[39] used a cutoff of one half of the total methodological score in their critical review of the effectiveness of conservative treatment for LBP. Thirteen studies were given a score of 45 points or more, and we deemed these studies to be methodologically acceptable. The most common methodological problems identified in this group of studies were the following: (1) failure to provide validity and reliability data on the survey questions that participants were asked, (2) lack of precise estimates (95% confidence intervals) of the prevalence rates provided, and (3) no comparison of study participants and nonparticipants to determine the generalizability of the sample obtained.

[TABULAR DATA 5 NOT REPRODUCIBLE IN ASCII]

Prevalence Estimates

Figure 2 shows the point prevalence of LBP from studies that were identified as methodologically acceptable (ie, scored [is greater than or equal to] 45 points) and provided point prevalence estimates. The 95% confidence intervals are shown for the studies by Cassidy et al[26] and Hillman et al.[29] Confidence intervals were not provided by other authors and thus are not shown. Prevalence estimates are further grouped by duration of LBP for comparison. The duration of LBP was not provided in the study by Skovron et al[35] and is marked as unknown. Prevalence estimates varied from study to study. Generally, studies in which LBP was examined on the survey day have higher prevalence rates than studies dealing with the seriousness of LBP or LBP lasting greater than 2 weeks. The point prevalence rates in this group of studies varied-from 4.4% to 33.0% ([bar]X=19.2%, SD=9.6%). Figure 3 shows the 1-year prevalence rates summarized by study. Again, a range of rates is presented (3.9%-63%). In these methodologically acceptable studies, the mean was 32.37% (SD=23.6%). The trend of higher rates for shorter duration of LBP examined is seen in both period and point prevalence estimates.

[Figures 2-3 ILLUSTRATION OMITTED]

Point prevalence estimates by age, when available, from the studies deemed methodologically acceptable are presented in Figure 4. The results suggest that younger subjects tend to have lower overall prevalence rates compared with older subjects up to the age of 60 years. Beyond the age of 60 years, it appears that prevalence rates decline.[7,28]

[Figure 4 ILLUSTRATION OMITTED]

Discussion

A large variation in community prevalence rates was observed among methodologically acceptable studies. The duration of LBP applied to define a prevalent case varied among studies, and this feature appeared to explain much of the variation in the reported prevalence rates. For example, if a person had an episode of LBP lasting only several days, this would not count as a prevalent case of LBP in a study investigating the prevalence of LBP lasting at least 2 weeks. There is also evidence that qualifying statements about duration may improve the accuracy of reporting.[40] Long-term memory is related to the duration of a painful experience and the frequency with which it is reoccurring.[40] In our opinion, therefore, definitions of the duration and severity of LBP need to be standardized for adequate comparisons among studies.

A second reason for the range of prevalence rates that we observed is the varying methodological quality among studies, as indicated by the scores obtained. The cutoff of 45 points or more as methodologically acceptable seems generous to us. The point prevalence rates in the 3 studies[22,26,29] that were given very high quality scores ([is greater than or equal to] 70) were 13.7%, 28.7%, and 19%, respectively. One-year prevalence rates were similar in the studies of Biering-Sorensen[22] (44.9%) and Hillman et al[29] (39%). Cassidy et al[26] reported a 6-month prevalence rate of 68.8%. In these studies, similar definitions were used for duration of LBP. We feel more confident in making conclusions about LBP prevalence from these studies because of their methodological strength.

The disparity in LBP prevalence rates among studies of similar methodological rigor, although smaller in methodologically superior studies, leads us to consider factors other than methodological issues to help explain differences. Age appears to be one factor influencing the prevalence rates in the studies reviewed. The methodologically acceptable studies that provided subgroup estimates by age demonstrated varying LBP point prevalence with age. The younger patients (aged 20-35 years) had lower prevalence rates in all studies reviewed. Prevalence estimates then increased in the middle years (ages 40-60 years), with the exception of the group surveyed by Biering-Sorensen.[22] After 60 years of age, the point prevalence rates tended to decline again. Deyo and Tsui-Wu[7] stated that there may be a cohort effect, such that older people simply experience less back pain than younger people do. Selective mortality and poorer recall also may explain the fall in prevalence rates among people over 60 years of age.[7] The overall prevalence rates observed in studies[7,33] that included younger subjects were lower than those of studies that examined only subjects in their middle years.

A major methodological challenge in determining the prevalence of LBP in the general population is that there is no standardized tool for diagnosis. Radiographs, computed tomography scans, and magnetic resonance imaging have aided with diagnosis, but they do not always indicate the cause. Thus, we believe there is a need to rely on other indexes of disease such as patient reports of LBP.[35] Reports of pain, whether by phone or by interview, can lead to inaccurate results. Biering-Sorensen and Hilden[40] reported that subjects responding to a mailed questionnaire consistently answered questions of ever having LBP 84% of the time on repeated occasions. Walsh and Coggon[41] also examined the repeatability of a self-administered questionnaire and found agreement of 89% among reports of LBP. The associated error is likely due to recall bias. Carey et al[21] suggested that false reporting was minimal (3%). Thus, until a standardized diagnostic tool is developed, substantial measurement error in prevalence rates is likely and rates will vary among studies using different durations of pain.

Other methodological challenges include using survey questions that have been shown to have sound measurement properties. Many authors of the studies we reviewed did not provide reliability and validity data for the questions used. Leboeuf-Yde et al[32] tested the LBP questions in their study and provided references of reliability and validity. Cassidy et al[26] used a questionnaire with what we consider acceptable reliability and validity. Choosing the method of questionnaire administration also will influence outcome.[40] Telephone interviews and self-administered questionnaires are more feasible than personal interviews, but they may not be the most accurate.[16] Finally, obtaining high response rates in population surveys can be a challenge. We contend that it is imperative that comparisons of respondents and nonrespondents be provided so that the generalizability of the study sample can be determined.

Conclusion

Future research on the community prevalence of LBP is needed before an accurate assessment of the societal impact of LBP on society with respect to disability and cost can be determined. Improved methodological quality and homogeneity among researchers are needed in all areas identified above. Generalizations to the community can be made with confidence from the 3 studies that we deemed to be methodologically superior. The point prevalence rate was estimated to be 13.7% in Glostrup, Denmark[22]; 28.7% in Saskatchewan, Canada[26]; and 19% in Bradford, United Kingdom.[29] The prevalence of LBP in North America was estimated by 3 studies reviewed.[7,26,33] The mean point prevalence rate estimated by Deyo and Tsui-Wu[7] and Lee et al[33]was 5.6%. This estimate is considerably lower than the estimate by Cassidy et al[26] (28.7%), likely because of the definition of duration of LBP used in each study. Cassidy et al[26] investigated LBP on the day of the survey, whereas Deyo and Tsui-Wu[7] and Lee et al[33] examined the seriousness of LBP and LBP of greater than 2 weeks' duration, respectively. Using the estimate of 5.6%, the population of North American adults is roughly 178 million.[42,43] Thus, roughly 10 million people are experiencing LBP on any given day. Many of these individuals will need medical care. Thus, it appears justified to contend that further research on the models of care and the effectiveness of treatments lot LBP is needed and that more accurate prevalence estimates would aid in that research.

References

[1] Frank JW, Kerr MS, Brooker A, et al. Disability resulting from occupational low back pain: I: What do we know about primary prevention? A review of the scientific evidence on prevention before disability begins. Spine. 1996;21:2908-2917.

[2] Harreby M, Neergaard K, Hellelsoe G, Kjer J. Are radiologic changes in the thoracic and lumbar spine of adolescents risk factors for low back pain in adults? A 25-year prospective cohort study of 640 school children. Spine. 1995;21:2298-2302.

[3] Szpalski M, Nordin M, Skovron ML, et al. Health care utilization for low back pain in Belgium: influence of sociocultural factors and health beliefs. Spine. 1995;20:431-42.

[4] Jekel J, Elmore J, Katz D. Epidemiology, Biostatistics, and Preventative Medicine. Toronto, Ontario, Canada: WB Saunders Co; 1996:20-22.

[5] Fletcher R, Fletcher 5, Wagner E. Clinical Epidemiology: The Essentials. 2nd ed. Baltimore, Md: Williams & Wilkins; 1988:76-87.

[6] Gordis L. Epidemiology. Toronto, Ontario, Canada: WB Saunders Co; 1996:32-34.

[7] Deyo RA, Tsui-Wu Y. Descriptive epidemiology of low-back pain and its related medical care in the United States. Spine. 1987;12:264-268.

[8] Leboeuf-Yde C, Lauritsen JM. The prevalence of low back pain in the literature: a structured review of 26 Nordic studies from 1954 to 1993. Spine. 1995;20:2112-2118.

[9] Volinn E. The epidemiology of low back pain in the rest of the world: a review of surveys in low- and middle-income countries. Spine. 1997;22:1747-1754.

[10] Loney PL, Chambers LW, Bennett KJ, et al. Critical appraisal of the health care literature: how to critically appraise an article about prevalence or incidence of a health problem. Chronic Diseases in Canada. In press.

[11] Frank JW, Brooker A, DeMaio SE, et al. Disability resulting from occupational low back pain, II: What do we know about secondary prevention? A review of the scientific evidence on prevention after disability begins. Spine. 1996;21:2918-2929.

[12] Bogduck N, Twomey LT. Clinical Anatomy of the Lumbar Spine. New York, NY: Churchill Livingstone Inc; 1987:5.

[13] Counsell C, Fraser H. Identifying relevant studies for systematic reviews. BMJ. 1995;310:126.

[14] Spoor P, Airey M, Bennett C, et al. Use of the capture-recapture technique to evaluate the completeness of systematic literature searches. BMJ. 1996;313:342-343.

[15] Bishop YM, Fienberg SE, Holland PW. Discrete Multivariate Analysis: Theory and Practice. Cambridge, Mass: The MIT Press; 1975:229-256.

[16] Streiner DL, Norman GR. Health Measurement Scales: A Practical Guide to Their Development and Use. 2nd ed. New York, NY: Oxford University Press Inc; 1995:85-88.

[17] Dixon RA, Munro JF, Silcocks PB. The Evidence-Based Medicine Workbook: Critical Appraisal for Clinical Problem Solving. Boston, Mass: Butterworth-Heinemann; 1997:199-206.

[18] Rockwood K, Stadnyk K. The prevalence of dementia in the elderly: a review. Can J Psychiatry. 1994;39:253-257.

[19] Kachigan S. Statistical Analysis: An Interdisciplinary Introduction to Univariate and Multivariate Methods. New York, NY: Radius Press; 1986:158-159.

[20] Marshall V. Factors affecting response and completion rates in some Canadian studies. Canadian Journal of Aging. 1987;6:217-227.

[21] Carey TS, Garrett J, Jackman A, et al. Reporting of acute low back pain in a telephone interview: identification of potential biases. Spine. 1995;20:787-790.

[22] Biering-Sorensen F. Low back trouble in a general population of 30-, 40-, 50-, and 60-year-old men and women: study design, representative years, and basic results. Dan Med Bull. 1982;29:289-299.

[23] Brattberg G, Thorslund M, Wikman A. The prevalence of pain in a general population: the results of a postal survey in a county of Sweden. Pain. 1989;37:215-222.

[24] Carey TS, Evans AT, Hadler NM, et al. Care-seeking among individuals with chronic low back pain. Spine. 1995;20:312-317.

[25] Carey TS, Evans AT, Hadler NM, et al. Acute severe low back pain: a population-based study of prevalence and care-seeking. Spine. 1996; 21:339 -344.

[26] Cassidy JD, Carroll LJ, Cote P. The Saskatchewan Health and Back Pain Survey: the prevalence of lowback pain and related disability in Saskatchewan adults. Spine. 1998;23:1860-1867.

[27] Harreby M, Kjer J, Hesselsoe G, Neergaard K. Epidemiological aspects and risk factors for low back pain in 38-year-old men and women: a 25-year prospective cohort study of 640 school children. Eur Spine J. 1996;5:312-318.

[28] Heliovaara M, Sievers K, Impivaara O, et al. Descriptive epidemiology and public health aspects of low back pain. Ann Med. 1989;21:327-333.

[29] Hillman M, Wright A, Rajaramam G, et al. Prevalence of low back pain in the community: implication for service provision in Bradford, United Kingdom. J Epidemiol Community Health. 1996;50:347-352.

[30] Jacobsson L, Lindgarde F, Manthorpe R. The commonest rheumatic complaints of over six weeks' duration in a twelve-month period in a defined Swedish population: prevalences and relationships. Scand J Rheumatol. 1989;18:353-360.

[31] Lau EM, Egger P, Coggon D, et al. Low back pain in Hong Kong: prevalence and characteristics compared with Britain. J Epidemiol Community Health. 1995;49:492-494.

[32] Leboeuf-Yde C, Klougart N, Lauritzen T. How common is low back pain in the Nordic population? Data from a recent study on a middle-aged general Danish population and four surveys previously conducted in the Nordic countries. Spine. 1996;21:1518-1526.

[33] Lee P, Helewa A, Smythe HA, et al. Epidemiology of musculoskeletal disorders (complaints) :and related disability in Canada. J Rheumatol. 1985;12:1169-1173.

[34] Papageorgiou AC, Croft PR, Ferry S, et al. Estimating the prevalence of low back pain in the general population: evidence from the South Manchester Back Pain Survey. Spine. 1995;20:1889-1894.

[35] Skovron MI,, Szpalski M, Nordin M, et al. Sociocultural factors and back pain: a population-based study in Belgian adults. Spine. 1994;19: 129-137.

[36] Svensson H, Andersson GB. Low back pain in forty- to forty-seven-year-old men, I: frequency of occurrence and impact on medical services. Scand J Rehabil Med. 1982;14:47-53.

[37] Svensson H, Andersson GB, Johansson S, et al. A retrospective study of low-back pain in 38- to 64-year-old women: frequency of occurrence and impact on medical services. Spine. 1988;13:548-552.

[38] Walsh K, Cruddas M, Coggon D. Low back pain in eight areas of Britain. J Epidemiol Community Health. 1992;46:227-230.

[39] van Tulder MW, Koes BW, Bouter LM. Conservative treatment of acute and chronic nonspecific low back pain: a systematic review of randomized controlled trials of the most common interventions. Spine. 1997;22:2128-2156.

[40] Biering-Sorensen F, Hilden. J. Reproducibility of the history of low-back trouble. Spine. 1984;9:1280-286.

[41] Walsh K, Coggon D. Reproducibility of histories of low-back pain obtained by self-administered questionnaire. Spine. 1991;16:1075-1077.

[42] Statistics Canada. Available at: http://www.statcan.ca.

[43] US Bureau of the Census. Available at: http://www.census.gov.

Appendix.

Worksheet for Critical Assessment of an Article Determining Prevalence or Incidence

Title: --

Authors: --

Research Question: --

QUESTION                                COMMENTS

1. Was the design appropriate for the   Prevalence: survey,
research question?                        cross-sectional
                                        Incidence: prospective,
[] Yes                                    longitudinal
[] No
[] Cannot tell

2. Were setting of study and subjects   Were subjects representative
described in detail?                      of an appropriate
                                          population?
[] Yes
[] No
[] Cannot tell

2b. Are the subjects comparable to my
population of interest?

[] Yes
[] No
[] Cannot tell

3. Was subject sample obtained          What is the method of
appropriately?                            sampling? Was random
                                          sampling used (stratified,
[] Yes                                    if appropriate)?
[] No                                   Were eligibility
[] Cannot tell                            criteria appropriate?
                                        Were subjects identified
                                          from an appropriate
                                          group (eg, census
                                          data)?
                                        Were there referral
                                          patterns and referral
                                          filter bias?

4. Was sample size appropriate?         Was the sample large enough
                                          to have narrow confidence
[] Yes                                    limits for small
[] No                                     proportions?
[] Cannot tell

5. Were objective and appropriate       Are measures reliable and
criteria used for measurement of          valid and clinically
outcome?                                  appropriate?
                                        Are they a gold standard
[] Yes                                    of diagnosis?
[] No
[] Cannot tell

6. Was outcome measured
appropriately?                           -- blind
                                         -- unbiased
[] Yes                                   Were interviewers trained?
[] No
[] Cannot tell

7. Are the estimates of prevalence      Were descriptive statistics
precise?                                  and confidence intervals
                                          documented?
[] Yes                                  Are results broken into
[] No                                     subgroups using
[] Cannot tell                            appropriate
                                          prognostic factor?

8. Was response rate adequate?          Were refusals and
                                          incompletes recorded and
[] Yes                                    kept to a minimum?
[] No                                   Were all subjects randomly
[] Cannot tell                            selected and accounted
                                          for?
                                        Is their present clinical
                                          status known?
                                        Was follow-up long enough
                                          (incidence)?

PL Loney, BHSc(PT), BA, is Master of Science Degree Candidate, Health Research Methods Program, McMaster University, Hamilton, Ontario, Canada.

PW Stratford, PT, is Associate Professor, School of Rehabilitation Science, and Associate Member, Department of Clinical Epidemiology and Biostatistics, McMaster University. Address all correspondence to Mr Stratford at Faculty of Health Sciences, School of Rehabilitation Science, McMaster University, OT/PT Bldg T-16, 1280 Main St W, Hamilton, Ontario, Canada L8S 4K1 ([e-mail]).

Concept and research design, data collection, and project management were provided by Loney and Stratford; writing and data analysis, by Loney; institutional liaisons and manuscript review, by Stratford.

This article was submitted June 15, 1998, and was accepted January 13, 1999.