Systematic sampling selects units from an ordered population at a fixed interval after a random start. If the population
size is 
and a sample of size 
is wanted, choose 
uniformly from 
and select
 |
Each population unit then has inclusion probability 
,
although the selected units are not independent.
Systematic sampling is simple to administer and spreads the sample across the ordered list. It can have high sampling variance, and an individual sample can be unrepresentative, if the ordering has a periodic pattern aligned with the sampling interval. Variance estimation from a single systematic sample generally requires assumptions about the ordering.
