Since true random sampling is virtually impossible to achieve, the frequency of a haplotype is estimated from the number of times x that it appears in a database of size n (generally known as the "counting method"), taking into account the uncertainty due to sampling error. The frequency is thus estimated by (x+2)/(n+2) according to Balding and Nichols, and represented as "match probability" in the mtDNAmanager.
Balding DJ, Nichols RA. DNA profile match probability calculation: how to allow for population stratification, relatedness, database selection and single bands. Forensic Sci Int. 1994;64(2-3):125-140.