Systematic sampling is when a researcher selects every nth person on the sampling frame to be part of the sample. The nth number is selected by dividing the target population size (the number in the sampling frame) by the desired sample size. The most representative samples from this method are achieved when the sampling frame list is randomised (this is known as systematic random sampling) although the "quasi-random" sampling of picking every nth person however it is listed (e.g. alphabetically or by address) is still likely to produce a more representative sample than something like snowball sampling or opportunity sampling. However, as with random sampling, although it is unlikely, it is possible that you could have some social groups significantly over or under represented, which is why some sociologists prefer to introduce quotas or stratification (see quota sampling and stratified sampling).