Printer Friendly

Balancing the costs of forecasting errors in parole decisions.

I. INTRODUCTION

Parole decisions can be based on a variety of factors. Some are automatically considered and closely prescribe the actions to be token. Among these are mandatory releases after a full prison term has been served. Other factors can be discretionary. They may or may not be considered, and their use is subject to interpretation.

Among the discretionary factors commonly weighed are two fundamental kinds of risk. One risk is that an individual will be released only to commit new crimes, sometimes very serious ones. The other risk is that an individual will not be released although behavior on parole would have been exemplary. Since the 1920s, parole boards in the United States have tried to minimize both kinds of risks by constructing forecasts of how individuals will fare under supervision in the community, and then using those forecasts to inform parole decisions. (1) The results of this enterprise are mixed. (2)

One reason for the mixed performance is a failure to appreciate that there are inevitable tradeoffs between the two kinds of parole risks. If one is more likely, the other is less likely. A related reason is that the consequences of the competing risks have almost universally been treated symmetrically. The costs of releasing an individual who then commits crimes are implicitly assumed to equal the costs of failing to release an individual who would have succeeded in parole. An important consequence is that the forecasts that follow build in that equivalence. (3) If the costs are not truly equal, the forecasts can be misleading, often badly so. (4)

In this paper, I will review ways to think about forecasting errors in parole decisions. The key issue will be how to properly address the consequences of the two kinds of forecasting errors, known more formally as false positives and false negatives. In the parole context, false positives can be individuals incorrectly projected to be poor parole risks. False negatives can be individuals who are incorrectly projected to be good parole risks. Over many parole decisions, both kinds of errors are virtually inevitable and when improperly considered, adversely affect the quality of those decisions.

II. FALSE POSITIVES, FALSE NEGATIVES, AND FORECASTING

For a didactic discussion of false positives and false negatives, it is helpful to work with archetypes. Popular culture can provide them. For this paper, therefore, one of the ongoing appeals of the Star Wars movies is the presence of good and evil, starkly represented respectively by Luke Skywalker and Darth Vader. (5) Darth Vader vaporized an entire planet populated by millions of peace-loving humanoids. (6) Luke Skywalker saved the federation of such planets. (7)

A. False Positives and False Negatives

In the extreme, at least, parole boards may need to ascertain whether individuals whose cases are being reviewed are more like Darth Vader or more like Luke Skywalker. Parole boards are often especially sensitive to violent crimes committed by individuals on parole, so their risk assessment instruments and procedures may be tuned to identify the Darth Vaders. When they succeed, the term "true positive" can be applied. When they fail, the term "false negative" can be applied. In a parallel fashion, if the risk instrument correctly identifies a Luke Skywalker, one can use the term "true negative." And when that identification is in error, the term "false positive" can be applied. The four possibilities are shown in Table 1, with the two kinds of errors in bold type. A case can fall in any one of the four cells.

Looking at Table 1, there is a simple way to avoid all false negatives: just proceed as if all cases were like Darth Vader. That is, use only the "Predict Darth Vader" column. One can see that all of the true Darth Vaders will be correctly identified. However, all of the true Luke Skywalkers will automatically become false positives. In a parallel fashion, one can see in Table 1 that there is a simple way to avoid all false positives: just proceed as if all cases were like Luke Skywalker. That is, use only the "Predict Luke Skywalker" column. One can see that all true Luke Skywalkers will be correctly identified. However, all true Darth Vaders will automatically become false negatives. Both solutions are simple because there is no need to do any forecasting. One rule or the other can be applied without any consideration of risk. But in practice, both solutions are likely to be unsatisfactory, in part because all differences between individuals are ignored. For example, first-time offenders are treated the same as habitual offenders. One response is to introduce forecasts of risk based on individual differences.

B. Parole Decisions as Parole Forecasts

At the time parole decisions are made, the consequences of parole decisions cannot be known. That is why forecasts are required even when they are not acknowledged as such. A real forecast is being made, often implicitly. If the forecasted outcome is favorable, the individual is more likely to be released. If the forecasted outcome is unfavorable, the individual is less likely to be released.

The basis of parole forecasts is necessarily historical. In effect, past cases serve as "training data." For these cases, the parole decision and subsequent outcomes can be known. Why cases fall in any of the four cells in Table 1 can be considered. Sometimes that information is used to construct systematic risk instruments. Sometimes it is used anecdotally. In both instances, the historical information informs current decisions that are also shaped by various legal requirements, and are often combined with behavioral evaluations derived from formal clinical training, research findings, theory from the social sciences, and a range of craftlore. For example, an inmate's "motivation" can be an important factor.

Because at the time parole decisions are made, one cannot know to which of Table 1's cells a case actually belongs, Table 1 is of no help with the instant case. Rather, Table 1 is an essential part of the process by which forecasting procedures are constructed from historical data. We turn to that next.

C. The Relative Costs of Forecasting Errors

In an ideal world, the forecasts would be perfect. There would be no false positives and no false negatives. In practice, there can be a substantial number of both. Moreover, there are tradeoffs between the two. If the net is cast to catch a larger number of true negatives, it will almost certainly catch a larger number of false positives as well. If the net is cast to catch a larger number of true positives, it will almost certainly catch a larger number of false negatives as well. This means that as false negatives decrease, false positives increase, and likewise, as false positives increase, false negatives decrease. How, then, should the tradeoff between false positives and false negatives be undertaken? The answer is that it depends on their relative costs.

When a case becomes a false positive or false negative, a forecasting error has been made, and there are costs associated with each. A false negative often means that a serious crime has been committed. There are one or more victims and various criminal justice agencies become involved. A false positive often means that an individual spends additional time in prison, which not only can be costly in monetary terms, but can have negative consequences for the individual and the individual's family. Obtaining credible quantitative costs for such forecasting errors can be difficult. Fortunately, it is also not necessary. All one needs is the relative costs: how much more or less costly a false negative is compared to a false positive.

One can arrive at the relative costs by deciding how many false positives one will accept for every false negative, or equivalently, how many false negatives one will accept for every false positive. This is really not much different from more familiar pricing mechanisms. If a consumer in a supermarket is indifferent between four boxes of cereal and one gallon of milk, a gallon of milk is worth four times more to that consumer than a box of cereal. Note that the absolute value of the cereal and the milk need not be determined to arrive at their relative value.

In the parole setting, if one can accept ten false positives for every false negative, false negatives are ten times more costly than false positives; one false negative counts the same as ten false positives. Conversely, if one can accept ten false negatives for every false positive, false positives are ten times more costly than false negatives; one false positive counts the same as ten false negatives. (8)

D. Building in Relative Costs

The relative costs of forecasting errors should be consciously built into the forecasts, and the reasoning rests on common sense. Which is worse: treating a Luke Skywalker like a Darth Vader or treating a Darth Vader like a Luke Skywalker? And then, how much worse? For example, if it is much worse to treat a Luke Skywalker like a Darth Vader, one wants to be very sure that a case is indeed a Darth Vader. Stated more formally, if false positives are much more costly than false negatives, then one should be quite sure before a case is forecasted as a Darth Vader. In contrast, if it is much worse to treat a Darth Vader like a Luke Skywalker, one wants to be very sure that a case is indeed a Luke Skywalker. Stated more formally, if false negatives are much more costly than false positives, then one should be quite sure before a case is forecasted as a Luke Skywalker. These ideas can be made quite precise.

Consider the following example. Suppose one has developed a risk scale for violent crime. That scale ranges from a probability of 1.0 that a violent crime will be committed, to a probability of 0.0 that a violent crime will be committed. At the extremes, one is certain that a violent crime will be committed or that a violent crime will not be committed. How this scale might be used is illustrated in Figure 1.

[FIGURE 1 OMITTED]

If the costs of false negatives and false positives are the same, an individual with a violent crime risk probability greater than 0.50 should be treated as a potential Darth Vader. An individual with a violent crime risk probability of 0.50 or less should be treated as a potential Luke Skywalker. (9) This is represented by the middle two-headed arrow in Figure 1. The threshold of 0.50 splits the arrow into two segments of equal length, implying that false negatives and false positives have equal costs. (10)

Suppose now that the costs of a false negative are taken to be three times the costs of a false positive. It follows that the violent crime risk threshold should be dropped and, in particular, the probability threshold should be set at 0.25 (0.75/0.25 = 3.0). This is represented by the top two-headed arrow in Figure 1. The threshold splits the two-headed arrow so that the right segment is three times longer than the left segment. Locating the threshold in this fashion captures more potential Darth Vaders, and is required by the three-to-one cost ratio. Put in other terms, the evidence hurdle that must be exceeded in order to treat an individual as a Darth Vader has been reduced by a factor of three compared to when the probability threshold was 0.50. Consequently, false negatives are reduced; fewer Darth Vaders are treated as if they were Luke Skywalker. The tradeoff, therefore, is that there will be more Luke Skywalkers treated as if they were Darth Vader--more false positives. But this is fully consistent with the three-to-one cost ratio.

Conversely, suppose the costs of a false negative are taken to be one-third the costs of a false positive. Now, the violent crime risk threshold is raised and in particular, the probability threshold should be set at 0.75 (0.25/0.75 = 1/3). This is represented by the bottom line in Figure 1. The threshold splits the two-headed arrow so that the right segment is one-third the length of the left segment. The threshold is set in this fashion to capture more Luke Skywalkers, as required by the one-to-three cost ratio. The evidence hurdle that must be exceeded to treat an individual as a Darth Vader has been raised by a factor of three, compared to when the probability threshold was 0.50. Consequently, false positives are reduced; fewer Luke Skywalkers are treated as if they are Darth Vader. The tradeoff is that more Darth Vaders will be treated as if they were Luke Skywalker--more false negatives. This is fully consistent with the one-to-three cost ratio. (11)

Looking back at Table 1, lowering the threshold has the effect of moving cases from the right column to the left column. Conversely, raising the threshold value has the effect of moving cases from the left column to the right column.

In summary, when parole forecasts are being made, one is, in practice, always trading false negatives against false positives. Even when these tradeoffs are not recognized or acknowledged, they are in play. And in those, tradeoffs are necessarily their relative costs. The evidence threshold can be moved up or down to take those relative costs into account. The usual assumption implicitly made is that the costs are identical, in which case the evidence threshold is located exactly in the middle of its range, favoring neither false positives nor false negatives. If this balance is not consistent with parole board preferences, the wrong evidence threshold is being used, and parole board preferences are being overridden. The good news is that the evidence threshold can be changed to be more consistent with parole board preferences.

III. DETERMINING RELATIVE COSTS

As already noted, it can be extremely difficult to accurately quantify the costs of forecasting errors. For example, how would one assign a dollar value to a heinous crime committed while on parole, or to the damage two extra years in prison can do to an inmate's employability? We have also seen that, fortunately, it is not necessary to quantify such costs. All that is required are relative costs. But, how can these relative costs be determined?

One approach is to ask explicitly how many false positives one can accept for every false negative, or how many false negatives one can accept for every false positive. (12) This seems like a manageable task. Nevertheless, arriving at such ratios can be challenging.

To begin, one must determine whose ratios matter. Who should be the relevant stakeholders? Presumably parole officials are in the mix. But what about former victims, the offender, or the offender's family? What about corrections officials, state legislators, or members of the state's executive branch? To the best of my knowledge, there has been no systematic discussion of such matters.

Another task is to elicit the cost ratios. Perhaps surprisingly, I have found that experienced criminal justice officials have little trouble when asked. Once reminded of the consequences that can follow from false positives and false negatives, they can rapidly decide "how much worse" one is than the other. For the jurisdictions I have worked in, parole officials treat false negatives as far more costly than false positives. Costs ratios of ten-to-one, or even twenty-to-one, are common. However, I have no experience with other kinds of stakeholders.

Even if the set of relevant stakeholders is relatively homogeneous (e.g., the state parole board), there will often be different views about what the ratio of false positives to false negatives should be. Then, the problem is how to work with the different cost ratios. One option is to average them, but this method can be unsatisfactory because some stakeholders' views should have more weight than others. For example, the view held by a parole board chair might be more important than the view held by any given parole board member. Unfortunately, weighting the ratios as part of the averaging introduces another messy complication. Precise weights are required, and it is not clear how such weights would be determined.

An alternative solution is to provide different forecasting procedures depending on the ratio of false positives to false negatives. This approach can help the process by which the implications of the forecasts can be addressed. At the very least, playing through the results of several different forecasting procedures, derived from several different cost ratios, could be a useful exercise to undertake every few years.

The best solution is for the stakeholders to arrive at a single consensus cost ratio. In my experience, this has not been difficult. The precise cost ratios usually articulated by individual stakeholders are not held with great conviction. The difference, for example, between a cost ratio of ten-to-one and five-to-one may not represent a principled disparity. In effect, the difference is noise. As a result, most stakeholders can readily compromise. Matters would, no doubt, be more contentious should the stakeholders be more heterogeneous.

A. Some Further Complications

In practice, there are several additional complications. First, whether an individual turns out to be a true positive or true negative depends heavily upon how behavior on parole is measured. Arrests, for instance, will miss crimes for which no arrest is made. Moreover, the arresting charge may not correspond well to the criminal behavior actually undertaken. Then, there is usually a translation into the categories used to characterize violations of parole. The hope is that those categories are useful, and yet broad enough to be sufficiently accurate. Thus, for parole violation purposes, it may not matter which kind of murder was committed. The individual has failed because of a violent crime.

Second, there may be a desire to have different cost ratios for different individuals. For example, the costs to an individual from additional time in prison can vary over time and over individuals. However, it will usually not be practical to arrive at a variety of cost ratios depending on the circumstances surrounding a given parole review. In addition to deciding what to do with the individual, the cost ratio would have to be determined. The time needed to make a parole decision could increase substantially. It would be more practical to determine a few different cost ratios in advance and apply them to broad categories of offenders. For example, the costs of incarceration are perhaps higher for individuals who have a wife and children.

Third, there are systemic issues that go beyond cost ratios surrounding particular individuals. A common problem is that if the cost ratio implies a large number of false positives, prison capacity may at some point be sorely taxed. That is, there is one cost ratio when there are plenty of prison beds, and a very different one when there are not. If changing circumstances such as these can be anticipated, they can be built into the forecasts. When the changes cannot be foreseen, there needs to be the capacity to rebuild the forecasting procedures taking the new circumstances into account.

Fourth, there must be clarity among decisionmakers about the consequences of overriding the forecasts. Overriding the forecasts can be useful when there is new information available that was not used when the forecasting procedures were developed. For example, the forecasting procedures may not have taken into account health conditions that could not be properly treated in prison hospitals, or which may imply the need for a parole on humanitarian grounds. In effect, the costs of a false positive are increased as new facts are brought to bear. In contrast, overriding a forecast because of a view that the existing information has not been properly evaluated will likely degrade the quality of the forecasts in an ad hoc manner. Since at least the mid-1970s, it has become clear that subjective judgments about risk and uncertainty are subject to a wide variety of biases, and that in general, statistical procedures can do a better job. (13) Subjective judgements informed by clinical expertise are compromised by the same sorts of problems. (14) Finally, the override may actually be an implicit effort to alter the cost ratio. If so, that effort should be addressed directly.

Fifth, there may be concerns that the cost ratios imposed have little empirical justification. That is, a complete cost analysis might produce very different results. On the one hand, that may not matter. What matters are the preferences of stakeholders, and a key feature of these preferences can be captured in elicited cost ratios. On the other hand, providing more thorough information about the relative costs of false negatives and false positives can usefully inform stakeholder preferences. In other words, the forecasts should respond to stakeholder preferences about the costs of forecasting errors, but well-informed preferences are surely desirable. If nothing else, forecast-informed parole decisions may be less subject to challenge.

Finally, there is only one way to reduce both false negatives and false positives: improve the performance of the forecasting procedures. This is not the place to consider the issues, but over the past several years, there has been some real success using new statistical procedures coupled with the power of modern computers. Rather than trying to develop theoretically informed models based on factors that are assumed to affect behavior on parole, the new approaches are crassly empirical. They "mine" very large databases containing hundreds of variables to find patterns that can help in forecasting. (15) At least so far, these methods seem to perform far better than conventional modeling approaches. (16)

IV. AN EXAMPLE

It should be clear that if the cost ratio of false negatives to false positives (or equivalently vice versa) is properly taken into account, forecasts can be affected in a useful manner. To fix this idea, we turn to an example. The data are comprised of a little over 10,000 individuals for whom parole forecasts were needed. The outcome is an arrest for any violent crime committed within eighteen months of release on parole. For this sample of offenders, around ten percent are arrested for a violent crime within eighteen months of release on parole.

From a statistical perspective, a violent crime for this population is a rare event. As such, it is difficult to find predictors to improve overall forecasting accuracy. In this instance, if one simply forecasted that no violent crime would be committed within eighteen months of release on parole, that forecast would be correct about ninety percent of the time. However, this is probably unsatisfying because all of the true positives would be false negatives--all of the Darth Vaders forecasted as Luke Skywalkers. In effect, one is assuming that false positives are far more costly than false negatives. In practice, it is likely that the reverse would be true. The simple forecast is not responsive to the likely cost ratio. It is important, therefore, to try to do better.

For this example, most of the predictors come from the usual sources that parole officials consult when they make parole decisions. Therefore, the usual predictors are included (e.g., age, gender, marital status, prior record). There are also some predictors that are often not available (e.g., behavior in prison, juvenile record). A machine learning procedure called Random Forests, implemented in the programming language R, was applied. (17) Random forests have been effective in other related work. (18)

Table 2 shows the result when the cost ratio of false negatives to false positives is set to about three-to-one. The numbers in the middle two columns are counts of the number of cases that fall in each cell. The two numbers in the far right column are the percentage forecasted correctly in each row. The top row is for individuals who are not arrested for violent crimes on parole. The bottom row is for individuals who are arrested for violent crimes on parole. All of the numbers in the table are the result of real forecasts. They are derived from data not used to build the forecasting algorithm.

One can see that about eighty percent of the negatives (i.e., no failure) are correctly forecasted; given that an individual did not commit a violent crime, the statistical procedures correctly forecast that outcome eighty times out of one hundred. About forty percent of the positives (i.e., failure) are correctly forecasted; given that an individual committed a violent crime, the statistical procedure correctly forecasts the outcome forty times out of one hundred.

Because for these data approximately ninety percent are not arrested for a violent crime within eighteen months of release, it is not surprising that such cases are identified with substantial accuracy. The performance for the positives is disappointing by comparison. However, one must not forget that arrests for violent crimes are relatively uncommon. Then, the improvement from a base rate of ten percent to forty percent looks better. That is, in the overall sample, the failure rate is about ten percent. In the subset identified by the forecasts, the failure rate is about forty percent, a four-fold improvement.

Table 3 shows the results when the cost ratio of false negatives to false positives is set to thirty-to-one. False negatives are taken to be far more costly than false positives and are given more weight as the forecasts are constructed. In effect, the threshold to identify a positive has been lowered substantially. One result is that there should be more false positives.

In Table 3, the negatives are forecasted with only about a forty-two percent accuracy, and the positives are forecasted with about an eighty-four percent accuracy. Performance for the negatives has been cut approximately in half. But, within the subset of offenders who were actually arrested for violent crimes, well over eighty percent can be forecasted correctly. At the same time, there are costs. There are about seven false positives for every true positive (5486/781). When the cost ratio was set to three-to-one, there were about five false positives for every true positive. Just as expected, raising the relative cost of false negatives results in more false positives.

The two cost ratios were set to provide a likely lower and upper bound for those ratios. In practice, something in between is a likely choice. It is also important to stress that both sets of results are formally correct. The underlying data are identical, and no technical errors have been made. Table 2 differs substantially from Table 3 because the imposed cost ratios differ dramatically. The results that would be more responsive to the needs of stakeholders depends on the cost ratio they favor. And that is a policy decision to be made before the data are analyzed. It is a constraint imposed on the forecasts.

Finally, there is extensive and informative output from random forests that would ordinarily be examined. In particular, there are measures of each predictor's unique contribution to forecasting accuracy. There are also plots of the functional form through which each predictor operates on the forecasts. Although both kinds of output can be very instructive, they are a diversion for this paper. Nevertheless, it is useful to add that one strong feature of random forests is that all of its output takes the cost ratio of forecasting errors into account. This is not true of most other methods.

V. CONCLUSIONS

Forecasting errors are part of the forecasting process, and all forecasting errors have costs. To ignore these costs is to accept a default that the costs of false negatives are the same as the costs of false positives. Forecasts build in this equivalence. When equal costs are inconsistent with the preferences of stakeholders, forecasts assuming equal costs can be very misleading. The solution is to elicit stakeholders' preferences for the relative cost of false negatives and false positives, and to allow the cost ratio to affect the forecasts. In some jurisdictions, this is being done.

(1) Howard G. Borden, Factors for Predicting Parole Success, 19 J. AM. INST. CRIM. L. & CRIMINOLOGY 328-36 (May 1928-Feb. 1929); see also Ernest W. Burgess, Factors Determining Success or Failure on Parole, in THE WORKINGS OF THE INDETERMINATE SENTENCE LAW AND THE PAROLE SYSTEM IN ILLINOIS 205-49 (Patterson Smith 1968).

(2) See generally Charles W. Dean & Thomas J. Duggan, Problems in Parole Prediction: A Historical Analysis, 15 SOC. PROBS. 450-459 (Spring 1968) (discussing why there was no "appreciable increase in predictive power" regarding parole outcome from the 1920s until the 1960s); see also David P. Farrington, Predicting Individual Crime Rates, in 9 PREDICTION AND CLASSIFICATION: CRIMINAL JUSTICE DECISION MAKING 53-102 (Don M. Gottfredson & Michael Tonry eds., 1987) (discussing predictions of individual crime rates, and the practical issues in applying such predictions to a penal policy); Stephen D. Gottfredson & Laura J. Moriarty, Statistical Risk Assessment: Old Problems and New Applications, 52 CRIME & DELINQ. 178-200 (2006) (discussing the use of "[s]tatistically based risk assessment devices" and why "[t]heir promise remains largely unfulfilled"); Richard Berk, Forecasting Methods in Crime and Justice, 4 ANN. REV. L. & SOC. SCI. 219-38 (2008) ("review[ing] modern forecasting methods and their applications to crime and justice questions").

(3) Richard A. Berk & Thomas F. Cooley, Errors in Forecasting Social Phenomena, 11 CLIMATIC CHANGE (SPECIAL ISSUE) 247-65 (1987) ("examin[ing] the nature of forecasting errors associated with social phenomena" and discussing solutions to increase accuracy).

(4) Richard Berk, Asymmetric Loss Functions for Forecasting in Criminal Justice Settings, 27 J. QUANTITATIVE CRIMINOLOGY (forthcoming Mar. 2011) (discussing "when the costs of forecasting errors are asymmetric and an asymmetric loss function is properly deployed, forecasts will more accurately inform the criminal justice activities than had a symmetric loss function been used instead').

(5) STAR WARS: EPISODE IV--A NEW HOPE (Twentieth Century Fox Film Corporation 1977).

(6) Id.

(7) Id.

(8) In economics, the cost of a product is not necessarily the same as the price of a product. The former speaks to the value used up when the product is made. The latter speaks to the value at which that product is sold. But parole decisions are not undertaken in a market, so the term "cost" (or "loss") will be used.

(9) What is done with individuals whose risk is exactly 0.50 does not matter in practice. Those individuals can be placed on either side of the 0.50 threshold. In effect, the two outcomes are equally likely, and one could just as well flip a coin.

(10) As the probability of being a Darth Vader drops, the probability that an individual is not a Darth Vader necessarily increases. In this example, that is the same as saying the probability that an individual is a Luke Skywalker increases.

(11) Any cost ratio can be represented in this fashion. The one-to-one, three-to-one, and one-to-three ratios are just illustrations.

(12) Alternatively and equivalently, one can ask how many false positives one can accept for every true positive, or how many false negatives one can accept for every true negative. Sometimes this is an easier method to implement, but it is a detail that will not be discussed here.

(13) See, e.g., Amos Tversky & Daniel Kahneman, Judgment Under Uncertainty: Heuristics and Biases, 185 SCI. 1124, 1130 (1974) (discussing heuristic biases on probability judgments).

(14) ROBYN DAWES, HOUSE OF CARDS: PSYCHOLOGY AND PSYCHOTHERAPY BUILT ON MYTH 27 (1994) (discussing the effect of biased judgments on clinical psychology); see also Joachim I. Kreuger, A Psychologist Between Logos and Ethos, in RATIONALITY AND SOCIAL RESPONSIBILITY: ESSAYS IN HONOR OF ROBYN MASON DAWES 1, 3 (Joachim I. Kreuger ed., 2008) (characterizing House of Cards as "the authoritative application of the principles of judgment and decision making to [clinical psychology]").

(15) RICHARD A. BERK, STATISTICAL LEARNING FROM A REGRESSION PERSPECTIVE vii--viii, 332-33 (discussing the benefits and application of statistical learning).

(16) Richard Berk et al., Forecasting Murder Within a Population of Probationers and Parolees: A High Stakes Application of Statistical Learning, 172 J. ROYAL STAT. SOC'Y: SERIES A 191, 196-97 (2009) (internal citations omitted) ('There are no classifiers to date that will consistently classify and forecast more accurately than [the] random forests [method]"; "[t]he random forests method is an inductive 'statistical learning' procedure that arrives at forecasts by aggregating the results from many hundreds of classification or regression trees.").

(17) Leo Breiman, Random Forests, 45 MACHINE LEARNING 5, 5-6 (2001) (discussing the concept and application of the random forests method).

(18) See, e.g., Berk et al., supra note 16, at 197, 208 (applying the random forests method to forecast the occurrence of murder within a population of probationers and parolees in Philadelphia); see also Richard Berk, The Role of Race in Forecasts of Violent Crime, 1 RACE & SOC. PROBS. 231, 233 (2009) (applying the random forests method in addressing "the role of race in forecasts of failure on probation or parole").

Richard Berk, Department of Statistics, Department of Criminology, University of Pennsylvania. Thanks go to Shawn Bushway and Jim Acker for very helpful comments on an earlier draft of this paper.
TABLE 1: PAROLE FORECASTING TYPOLOGY

 Predict Predict
 Darth Vader Luke Skywalker

Darth Vader True Positive False Negative
Luke Skywalker False Positive True Negative

TABLE 2: FORECASTING SKILL USING 3-TO-1 COST RATIO OF
FALSE NEGATIVES TO FALSE POSITIVE

 Predict Not Predict Percentage
 Violent Violent Correct

Not Violent 7607 1877 80%
Violent 561 376 40%

TABLE 3: FORECASTING SKILL USING 30-TO-1 COST RATIO
OF FALSE NEGATIVES TO FALSE POSITIVE

 Predict Not Predict Percentage
 Violent Violent Correct

Not Violent 3998 5486 42%
 Violent 156 781 84%
COPYRIGHT 2011 Albany Law School
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2011 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Author:Berk, Richard
Publication:Albany Law Review
Date:Mar 22, 2011
Words:5579
Previous Article:Foreword.
Next Article:Estimating empirical Blackstone ratios in two settings: murder cases and hiring.
Topics:

Terms of use | Privacy policy | Copyright © 2019 Farlex, Inc. | Feedback | For webmasters