The selection bias occurs in the case when the researcher has to make a decision on which participant to study. The selection bias is associated with those researches when the participant selection is not random. The selection bias is also called the selection effect. The selection bias is caused by as a result of the method of sample collection.

Four TYPES of selection bias are explained below:

1. Sampling Bias: As a result of a population that is not random at all, some members of a population have fewer chances of getting included than others, resulting in a biased sample. This causes a systematic error known as sampling bias.
2. Time interval: Trials may be stopped early if we reach any extreme value but if all variables are SIMILAR invariance, the variables with the highest variance have a higher CHANCE of achieving the extreme value.
3. Data: It is when specific data is SELECTED arbitrarily and the generally agreed criteria are not followed.
4. Attrition: Attrition in this context means the LOSS of the participants. It is the discounting of those subjects that did not complete the trial.