It consists in selecting from the general population the final, individual units of the study in a direct way, where all units of sampling are simultaneously units of the study. We divide simple random sampling into simple random sampling without replacement and simple random sampling with replacement. The simple random sampling scheme, although simple to use, has a number of drawbacks: 1) it is characterized by lower precision of inference compared to other schemes, especially when the population is internally differentiated; 2) does not use any information about the studied population; 3) usually entails high costs of developing a sampling frame and measurement (statistical observation) of randomly drawn units.

• Unit under observation
• Sample unit
• Sampling
• Systematic sampling

• Simple random sampling with replacement
• Simple random sampling without replacement
• general population