This is defined as process of partial enumeration that the sample units are selected in such a way that each and every unit of the population has an equal and independent chance of being selected in the sample.
Two conditions to select samples:
- With replacement: that sample unit is replaced before making next draw.
- Without replacement: that sample unit is not replaced before making next draw
Selection technique
- Lottery method - Lotto
- By random no. table method - Bingo
Merits
- Scientific method and no. bias
- Estimation methods are simple and easy
Demerits
- If sample chosen is widely spread, takes more time and cost
- A population frame or list is needed
- For a given precision, it usually requires larger sample size.
When to use
- If the population is not widely spread geographically
- If the population is more or less homogenous with respect to the characteristics under study
Most applicable for initial survey in an investigation.
