A simple random sample of individuals from a population is the easiest sampling scheme to understand, but other sampling schemes may give more accurate estimates of population characteristics.

Grouping of individuals

If the individuals in the population can be split into different groups (called strata in sampling terminology), it is often better to take a simple random sample within each separate group than to sample randomly from the whole population. This is called a stratified random sample.

The proportion sampled from the different strata are usually fixed to be the same as the proportions of individuals in the strata in the population. In a simple random sample, the proportions sampled from the strata might not match the population proportions, so a stratified random sample should be more 'representative'.