Sampling Methods

**Method** | **How it works** | **Main limitation** Simple random | Select from a complete population list so every possible sample of the fixed size has the same chance of selection. | An incomplete list excludes part of the population. Systematic | Choose a random start, then every kth individual. | A repeating pattern in the list can bias selection. Stratified | Divide into relevant groups, then randomly sample within every group. | Requires group membership and suitable group sizes. Quota | Recruit until specified group totals are met. | Selection within each group is not necessarily random. Convenience | Use individuals easiest to reach. | Easy access need not represent the population. Example: a school has 360 younger and 240 older students. A proportional stratified sample of 50 selects 30 and 20 respectively, randomly within each group.