The purported CSI effect and the retroactive precision fallacy

Regarding our recent post on the syllogism that ate science, someone points us to this article, “The CSI Effect: Popular Fiction About Forensic Science Affects Public Expectations About Real Forensic Science,” by N. J. Schweitzer and Michael J. Saks.

We’ll get to the CSI Effect in a bit, but first I want to share the passage from the article that my correspondent pointed out. It’s this bit from footnote 16:

Preliminary analyses (power analyses) suggested a sample size of 48 would be sufficient to detect the CSI effect, if it existed. In addition, the subsequent significance tests adjust for sample size by holding smaller samples to a higher standard when determining statistical significance. In other words, finding that a difference is statistically significant is the same as saying that the sample size was of sufficient size to test for the effect.

Emphasis added. This is a great quote because it expresses so clearly this error. What to call it? The “retroactive precision fallacy”?

For a skeptical take on the CSI effect, see this article by Jason Chin and Larysa Workewych, which begins:

The CSI effect posits that exposure to television programs that portray forensic science (e.g., CSI: Crime Scene Investigation) can change the way jurors evaluate forensic evidence. We review (1) the theory behind the CSI effect; (2) the perception of the effect among legal actors; (3) the academic treatment of the effect; and, (4) how courts have dealt with the effect. We demonstrate that while legal actors do see the CSI effect as a serious issue, there is virtually no empirical evidence suggesting it is a real phenomenon. Moreover, many of the remedies employed by courts may do no more than introduce bias into juror decision-making or even trigger the CSI effect when it would not normally occur.

My correspondent writes:

Some people were worried that the sophisticated version of CSI that is portrayed on TV sets up an unrealistic image and so jurors (who watch the show) will be more critical of the actual evidence, which is much lower tech. There have been a handful of studies trying to demonstrate this and two did (including the one at issue).

I was pretty shocked at the poor level of rigour across the board – I think that’s what happens when legal scholars (the other study to show the effect was done by a judge) try to do empirical/behavioural work.

The truly sad thing is that many courts give “anti-CSI Effect” instructions based on these two studies that seem to show the effect. Those instructions do seem to be damaging to me – the judge tells the jury that the prosecution need not bring forensic evidence at all. The number of appeals and court time spent on this shoddy line of research is also a bit problematic.

So, two issues here. First, is the CSI effect “real” (that is, is this a large and persistent effect)? Second, the article on the CSI effect demonstrates a statistical fallacy, which is the view that, once a statistically significant result has been found, that this retroactively removes all concerns about inferential uncertainty due to variation in the data.

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