What leads to selection bias error?

Selection bias error occurs when systematic differences exist between the participants selected for a study and those who are not, leading to an unrepresentative sample. This systematic difference can affect the results and conclusions drawn from the study because the sample may not accurately reflect the population being studied. For instance, if certain groups are more likely to be included in the research due to specific criteria or methods of outreach, the findings may not be applicable to the entire population.

Random assignment of participants typically helps mitigate selection bias by ensuring that each participant has an equal chance of being placed in any group, which contributes to the overall validity of the study. External factors influencing the data collection process or variability in measurement techniques can impact the reliability and validity of the data collected but do not directly contribute to selection bias itself. Hence, the presence of systematic differences in the selection process is the key factor leading to selection bias error.