Frequency of exclusion as the name itself suggests may refer to how often there is an occurrence in which a group or a sample is not included in a larger sample or study to determine some qualitative or quantitative metric about the population being studied.
The frequency of exclusion would make an attempt to calculate the ratio or more effectively a percentage to come up with a rate at which a particular group is not represented or highly under-represented in a sample for a study. The results obtained from these kinds of statistical studies or surveys become meaningless or are of no worth if the sample group is incapable of accurately representing the entire population which has been identified as the target sample.
As an example we can consider a case in which, one can come up with the percentage to calculate rate at which people employed in a particular sector are almost always excluded from a particular employment survey due to the work timing or working hours. If a certain employment sector is not properly represented in a fair amount in the research survey, then the effects of a policy or scheme will not show the real picture that would actually come up when the scheme is launched all over the country.