Ask any scientist and she'll tell you that the best result from an experiment is often the one that disproves the hypothesis being tested. Why? Because such a result means there are more questions to ask - and more knowledge to be gained.
According to the scientific method, we first develop a hypothesis, then we conduct experiments. We observe the data obtained and compare the results to our hypothesis. If the data contradicts the hypothesis we must change our hypothesis. That, you see, is the recipe for scientific breakthroughs.
But such a scenario is anathema in a crime lab.
The last thing the police want is an inconclusive test. Analysts know this. Their supervisors know this. The people funding the lab know this.
And so the "forensic scientists" at crime labs across the country are made part of the criminal investigation. They are given the task of proving a link between the test result and the suspect. And, as many results are subject to interpretation, there is an inherent bias to be found.
Grits for Breakfast referenced an article in The Economist that when forensic scientists are given too much "contextual information," test results can be subject to a "cognitive bias."
As Grits points out medical trials work because the doctors conducting them don't know who's receiving the actual treatment and who's receiving the placebo - and neither do the test subjects. Without that knowledge there is no pressure (whether real or imagined) to make the results fit the desired outcome.
In the criminal (in)justice context, however, the forensic scientists, or lab technicians, running the test are told they are testing something that came from the suspect. They are told what that person is suspected of doing. They are told they're part of the team. Their mission, in other words, is to provide the evidence the government needs to obtain a conviction.
They are neither neutral nor unbiased. Just think what role that ideology played in the expansion of forensic analysis into the analysis of tire tracks and foot prints. Fingerprints and bite marks. Ballistics and lead analysis. Arson investigations.
If we are going to call these glorified lab techs "forensic scientists," then we should at least expect them to play by the same rules other scientists follow. It's the least we should demand when people's lives are on the line.