Systematic Sampling Interval Calculator
Population ÷ sample size gives the interval. A random start plus that interval gives a selection anyone else could repeat exactly — which is what makes it defensible.
Systematic sample interval calculator
The interval for systematic selection, and the first few items it would select from a random start — write down the start, the interval, and every item, so someone else could repeat the selection exactly.
Educational illustration only. The random start should genuinely be chosen at random between 1 and the interval — entering a convenient number here defeats the purpose of random selection.
A worked example
A population of 2,000 invoices, numbered in order, and 25 are needed. The interval is 2,000 ÷ 25 = 80. Pick a random start between 1 and 80 — say 37. Select invoice 37, then 117, then 197, and so on, adding 80 each time.
When systematic selection backfires
Systematic selection is simple and fair, provided the list has no repeating pattern. If every tenth item happens to be a particular kind of entry (a month-end bulk invoice, for example), a systematic interval that happens to be a multiple of ten would land on the same kind of item every time — giving a badly distorted picture. Random selection avoids that specific failure mode.
See the full sampling method
This calculator covers one method from a larger framework — random, systematic, haphazard and targeted selection, and what actually drives sample size — taught in full in Advanced Audit & Assurance.