“”I'm an expert witness. Why? Because I say I am.
|—Satan, The Garden of Allah; Don Henley|
Sometimes, they do good work… sometimes, they don't. In general, an "expert" witness may be brought in ostensibly to present at least one argument from authority. This is independent of which side of the prosecution the expert witness is testifying. For example, in the murder trial of Sally Clarke, who was accused of killing both of her children, an expert witness was brought in to state that the odds against the children's deaths was 1 in 73 million. This seemed to imply that the odds of her being guilty of murder were 72,999,999 out of 73 million. For whatever reason, the jury issued a guilty verdict. It seemed beyond question that the probability argument weighed heavily in the minds of the jurors. The problem is, even experts are prone to cognitive biases, and likely to emphasize incriminating evidence, while ignoring the exculpatory evidence. In the Sally Clarke case, the 1 in 73 million figure was irrelevant in the case, as the two children were already dead, therefore it is only certain that the post hoc probability was equal to one.
- The Prosecutor’s Phallusy by Ben Goldacre (October 28, 2006) Bad Science.