DNA exoneration: what are the chances? As with any biometric, both the False Acceptance Rate (i.e. what is the chance that two samples of different individuals match) as well as the False Rejection Rate (i.e. what is the chance that two samples of the same individual do not match) are important.
However, in DNA studies, it seems most attention goes to the FAR. The FAR is important indeed: in court it is unacceptable that an innocent person would have a large chance to be convicted based on a DNA sample. However, the FRR is also important: it is unacceptable that a criminal walks because the DNA test falsely rejected the match. 
It seems that "DNA has an extremely low false acceptance rate, but an uncertain false rejection rate." (source: Cyber Crime: Concepts, Methodologies, Tools and Applications) 
Now, my question is: are there any studies that have investigated this FRR for DNA? What are the statistics on this? The 'Innocence project' mentions that some criminals have been exonerated based on a DNA test after their conviction. Are we sure that those exonerated criminials were indeed innocent, or were they just falsely rejected?
 A: Are there any studies that have investigated this FRR for DNA? 
Yes, what you are refering to is also called a type II error or a false negative. People have investigated this and also for lab work in general:


*

*Koehler et al.

*Kloosterman et al.

*Lapworth & Teal

*NFI
A very broad range of error values, with Koehler reporting a ridiculous 12 out of 1000.
What are the statistics on this?
If the test would have been a perfect test it would still have an error rate due to the presence of homozygotic twins in the population and the small chance of two people having identical DNA profiles, called a coincidental match ($\approx 1\cdot 10^{-9}\%$). There are approximatly 0.3% homozygotic twins in the population which gives an error rate of 0.15% in the case of a perfect test.
but...
Since the test is not completely perfect and humans aren't either, there is a larger error introduced. Like every other test there are type I errors which might be due to equipment, hetrogeneous DNA mixture, human error or some unknown external source. 
Whenever you design a test you also test the test on its error rate by testing some samples of which you know the outcome:
$$False\;negative\;rate= \frac{False\;negative}{False\;negative+True\;positives}$$
This would yield the value of the error rate and is different for different laboratories (due to different people, equipment, etc.). If you are into  bayes theorem and would like to know more about error statistics in forensic DNA test: Thompson et al. 
It is unacceptable that a criminal walks because the DNA test falsely rejected the match.
Indeed, but a good judge would not make a decision solely on a DNA test and bears in mind there are errors in testing. However when a DNA test is presented, the reported error rate is the chance of getting a coincidental match!
