"Linking PCBs to play behaviors of Dutch girls and boys": Vreugdenhil et al.'s response. (Correspondence).

Kaufman's comments on the findings we presented in our article "Effects of Prenatal Exposure to PCBs and Dioxins on Play Behavior in Children at School Age" (Vreugdenhil et al. 2002) is based on three points: the instrument, the age of the children, and the use of an interaction term in our analysis. We would like to respond to these questions.

The Pre-School Activities Inventory (PSAI) is a very simple parent questionnaire. We included the 24 questions in our paper in the form of an appendix. Our data on the PSAI show that boys scored significantly higher on the masculine scale than girls, and correspondingly, girls scored significantly higher on the feminine scale than boys. The composite scores for boys also indicated an overall masculine score, and correspondingly, the composite scores for girls indicated an overall feminine score. These data show that the PSAI is a valuable instrument in assessing masculine and feminine play behavior in Dutch boys and girls at 7 years of age. We agree with Kaufman that parent's perceptions might be biased. That we found significant differences in the effects of prenatal PCB exposure with such a relatively simple parent questionnaire in our exploratory study makes our findings even more relevant.

Because we were interested in effects of PCB exposure on masculine and feminine play behavior, we presented our results for the masculine, feminine, and composite (defined as the within-subject difference; feminine score minus masculine score) PSAI scores. We estimated the effect of PCB exposure on these scores and the difference in effect between males and females by fitting one regression model for each outcome variable (masculine, feminine, and composite score) in the combined data set of males and females. Because this was an observational study, we included a number of confounding variables in the linear model. The set of covariates taken along with exposure (the variable of interest) included type of feeding in infancy, duration of breast-feeding, sex, parity, parental education level, parental IQ, the home environment, and the age of the child. First, this means that through the variable "sex" the model allows for a bimodal distribution of the scores, so that the results are neatly and automatically adjusted for that bimodality. Indeed, as expected, the distribution of the residuals estimated from the regression analysis no longer shows that bimodal property. Second, through the variable age in the model, the estimated effects of exposure on the outcome variables have automatically become age-adjusted. Of course, this also means that the difference between boys and gifts in the exposure effect (as represented by the coefficient of the sex by the exposure--interaction term) is automatically adjusted for the sex-bimodality and for age, as well as for any of the other covariates in the model. Therefore, we do not see any of the methodological flaws mentioned by Kaufman, and we hope that we have hereby reassured Kaufman on this matter.

We presented the p-values of the statistical tests as calculated to three decimal places. Because of the exploratory nature of this observational study, we did not apply multiplicity correction on an overall significance level to obtain a significance level per test. We think that the scientific audience should be free to decide how much significance to attach to each result, given the corresponding reported p-value and prior knowledge of the results of other publications on this subject. We do not understand how the absence of a rather arbitrary multiplicity correction per test on an (also rather arbitrary) overall significance level has led Kaufman to try to convince others to place little confidence in our findings. How else can scientists build evidence unless we present our results, including exploratory studies.

The authors declare they have no conflict of interest.

REFERENCES

Vreugdenhil HJI, Slijper FME, Muider PGH, Weisglas-Kuperus N. 2002. Effects of perinatal exposure to PCBs and dioxins on play behavior in Dutch children at school age. Environ Health Perspect 110:A593-A598

Hestien J. I. Vreugdenhil

Erasmus University Rotterdam

and Sophia Children's Hospital

Rotterdam, the Netherlands

Paul G. H. Mulder

Erasmus University Rotterdam

Rotterdam, the Netherlands

Nynke Weisglas-Kuperus

Erasmus University Rotterdam

and Sophia Children's Hospital

Rotterdam, the Netherlands

E-mail: [e-mail]