Making Sense of Statistics in Court

I talked about making sense of statistical data in general a while back.  Recently, the latest issue of Nature ( vol.  445, no. 7125, p. 254-255 [electronic version requires subscription]) features a story of how statistics has been used and interpreted in court house.  I guess the author, Mark Buchanan, intended to remind to beware of the use of statistics outside scientific arena.  In social setting, nothing can be controlled as in an experiment room.  Context is crucially needed.  He introduced two particular cases.  The first one is suspected nurse who may have killed her patients.  The second one is a mother who has been suspected of killing her two daughters.  Both cases involved the probability of the chances that suspects could have committed murders.

Two points mentioned in the paper are standing out.  For the nurse’s case, the statistician told that “the chance that her presence was mere coincidence was only 1 to 342 million”.  Well that seems very impossible, right?  But the mathematician said by brining additional independent variable – include more factors – the chance could be as small as 1 in 48 or 1 in 5.  My question is which number seems possible for most of us then.  At what level of chance that you can say that could not be coincidence anymore?

Another point is from the quote below.

[L]awyers have an incentive, and even a duty, to select the evidence that makes their case stronger.  “What the judge ends up hearing often comes from the two extreme ends of the distribution,” he (David Kaye) said.

I think the use of statistics is very outstanding example of how people make sense of “number” is very influential and does affect the way we live.

Note:

Inside this issue of Nature (p.249), there is a article about Apple Inc. has been sued by its shareholder about backdating stock options.  They already confessed and Jobs knows about it.

5 responses to “Making Sense of Statistics in Court

  1. I strongly support the uses of statistics (and other “systematic guess works”, like neural network) for investigations, as they could vastly shorten the investigation time, i.e. cut out the unlikely cases early, so the investigating team can put more effort on something more likely.

    But, BIG “BUT”, to use theses numbers to judge something as serious as, say, criminal case … I think that’s too danger, … and probably that’s already a crime by itself.

    So what I can say is, while I believe that counting numbers could bring us to an evidence, the numbers itself could not be counted as an evidence.

  2. It is interesting that a lot of people want to know the likelihood of something could happen without considering that the number without context is dangerous.

    In Western court, the jury is the most important factor. It is interesting to see how these people make decision based on these number.

    The article already said that the BIG number (e.g. 342 million) is too convincing. However, if comparing 532 “billion”, will the former case not guilty anymore? This kind of question would rather be intensively investigated and publicized.

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