Systematic sample
A systematic sample orders the population, picks a random starting point, then selects every kth individual from that point onward.
Systematic sampling chooses one random start and then follows a fixed interval, which is handy when a full list is hard to number in advance. For example, standing at a store entrance and surveying every 10th customer after a random start is a systematic sample. The gap between selections, often written (the sampling interval), is set so the selections cover the whole population. It can fail if the list has a repeating pattern that lines up with the interval.
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