Each system has a certain number of repeats. From a statistical standpoint, we look at the frequency these repeats occur in each system. Therefore, in D21S11, if 32.2 is seen at a frequency of .0012 and a 29.3 is seen at a rate of .1236 the system frequency is calculated as follows:

2 (.0124)(.1236) = .00306, so when we look at several systems each system is multiplied using the product rule...so the frequency is extremely small. Let us say the eight other systems where hypothetically the same frequency then the number would be 2.3 x 10-23 or this can be expressed as a likelihood ratio by 1 / 2.3 x 10-23 or One in 41,857,050,000,000,000,000,000.