reporter: the test instructions say findings must be confirmed by a second laboratory test. but miller says in many cases, that s not happening. especially harmful, he says considering many of the tests are done at night. you can imagine that that situation can lead to a lot of sort of bad reads. reporter: of those 30,000 mistaken arrests, the study finds nearly 90% result in a wrongful conviction. at this point, it s the largest known specific single factor leading to wrongful convictions in the united states. reporter: is it that the tests themselves are flawed? or are the departments using them incorrectly? i don t think it s fair to say the tests themselves are flawed. these tests were never intended to be the final decision-maker in terms of whether or not somebody is carrying drugs. reporter: the two drug test companies cited in the report and demonstration did not respond to our request for comment. for simon and his wife, they were eventually able to get their rec