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Past performance is not an indicator of future returns. That, at least, is the advice given to investors. But can the likelihood of a person committing a crime be predicted by looking at his record? The answer, according to a team of clinical forensic psychologists, is that it cannot. Not only is risk prediction unreliable but, when applied to individuals rather than groups, the margins of error are so high as to render any result meaningless.

Making assumptions about individuals from group data is generally only reasonably safe when the variation within the group is small. Despite this, risk assessments are routinely used to help decide who should be locked up, who should undergo therapy and who should go free. Risk prediction is also set to be used to assess the threat posed by people ranging from terrorist suspects to potential delinquents.

Stephen Hart, of Simon Fraser University in British Columbia, Canada, and colleagues decided to determine how accurate the tests of risk assessment are when applied to individuals rather than groups. Typically the tests work by assigning a score to people depending on factors such as their age, the history of their relationships, their criminal past and the type of victims they have chosen. If someone's score places him in a group in which a known proportion has gone on to commit a crime on release from detention, then the risk that person will prove a recidivist is thought to be similar to the risk for the group as a whole.

The paper published by Dr Hart and his colleagues in last month's issue of the British Journal of Psychiatry focused on two popular tests that follow this logic. The first was a 12-item test designed to assess risk for general violence over periods of seven to ten years. The second was a ten-item test designed to assess risk for violence and sexual violence over periods of five to 15 years. The researchers have also assessed other tests used for predicting sexual offences and domestic violence.

They found that variations between members of the groups were very large. In one of the tests, for example, the standard estimate of the chances of members of the group sexually reoffending was put at 36% within 15 years. They calculated that the actual range was between 30% and 43% of the group, with a 95% confidence level. But calculating the average probability for a group is much easier than calculating the same probability for any individual. Thus, using standard methods to move from group inferences to individual ones, they calculated that the chance of any one person reoffending was in the range of 3% to 91%, similarly with a 95% confidence level. Clearly, the seemingly precise initial figure is misleading.¡£

The principle is not peculiar to psychology. It has been recognised by statisticians for decades. They call it the ecological fallacy (although this term captures broader subtleties, too). Medicine has also been confounded by statistically based procedures. Indeed, the technique is only really useful when the successes and failures are aggregated. A life-insurance company, for instance, could wrongly predict the life span of every person it insured but still get the correct result for the group.

×¢£¨1£©£º±¾ÎÄÑ¡×ÔEconomist, 06/02/2007

×¢£¨2£©£º±¾ÎÄÏ°ÌâÃüÌâÄ£·Â¶ÔÏóΪ2004ÄêÕæÌâText 4¡£

1£® What do the clinical forensic psychologists think of risk prediction?

  [A] Risk prediction fails in the stock market.

    [B] Risk prediction is always effective when being applied to groups.

    [C] Risk prediction is not dependable when it comes to individual behavior.

    [D] Rist prediction should be widely used in all fields.

2£® We can learn from the text that tests of risk assessment are _______.

  [A] longitudinal

    [B] very tricky

    [C] convincing enough

    [D] unreasonable

3£® Traditional view and that of Dr. Hart on risk assessment are _______.

  [A] identical

    [B] similar

    [C] complemetary

    [D] opposite

4£® According to Dr. Hart, using standard methods to predict individuals _______.

  [A] is as easy as using them for groups

    [B] yields ineffective statistics

    [C] can help attain precise results

    [D] might be influcenced by confidence level

5£® Which of the following statements is TRUE?

    [A] Ecological fallacy is a psychological phenomenon.

    [B] Statisticians have been dealing with ecological fallacy for a long time and have almost succeeded in figuring out a solution.

    [C] The statistics from a life insurance company on the longevity of a group of people should be reliable.

    [D] Risk prediction is proved to be of zero practical value.

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margin [`mB:dVin] n. Ò³±ßµÄ¿Õ°×£¬²îÊý            confidence level ÖÃÐŶÈ

assessment [E`sesmEnt] n. ÆÀ¹À£¬¹À¼Û              probability [9prCbE`biliti] n. ¸ÅÂÊ

routine [ru:`ti:n] adj. ÀýÐеģ»³£¹æµÄ               inference [`infErEns] n. ÍÆÂÛ

therapy [`WerEpi] n. ÖÎÁÆ                         initial [i`niFEl] adj. ×î³õµÄ, ³õʼµÄ

delinquent [di`liNkwEnt] n. ʧְÕß, Î¥·¨Õß          fallacy [`fAlEsi] n. ÃýÎó, ÃýÂÛ

proportion [prE`pC:FEn] n. ±ÈÀý, ¾ùºâ             confound [kEn`faund] vt. ʹ»ìÏý, ´ì°Ü

detention [di`tenFEn] n. ¾ÐÁô, ½û±Õ                 aggregate [`Agrigeit] v. ¾Û¼¯, ºÏ¼Æ

psychiatry [sai`kaiEtri] n. ¾«Éñ²¡Ñ§, ¾«Éñ²¡ÖÎÁÆ·¨    insure [in`FuE] vt. ¸ø...±£ÏÕ

domestic [dE`mestik] adj. ¼ÒÍ¥µÄ, ¹úÄÚµÄ

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