- Proportionate Stratified Random Sampling

Here, we divide each stratum in proportion to its representation in the whole population under study. For instance, random technique

- Disproportionate Stratified Random Sampling

In disproportionate stratified random sampling, we do not go by sampling fraction. The intent here is to ensure that all groups in the population find representation in the sample, irrespective of the proportion of their representation in the population.

- Optimal stratified sampling

In optimal stratified sampling, we form groups in the proportion to the standard deviation of the observations. This is also known as Neyman optimal allocation. The allocation becomes optimal as it considers the size of strata as well as variability within the population.