Final answer:
Selection bias in case-control and cohort studies occurs when the study's subjects are not a representative sample of the target population, which can lead to incorrect conclusions. For case-control studies, biased recall can introduce selection bias, whereas in cohort studies, the issue may arise from an unrepresentative or non-randomly selected initial cohort.
Step-by-step explanation:
Selection bias in case-control and cohort studies usually occurs when the subjects selected for the study do not represent the target population accurately. In case-control studies, this happens often because subjects report past behaviors that may be either forgotten or misrepresented, leading to a distorted association between the behavior and the condition in question. As for cohort studies, even though the prospectively collected data are generally more accurate, selection bias can still occur if the initial cohort is not representative of the larger population. This can happen if the characteristics of people entering the study are significantly different from those of the general population that the study is meant to reflect, or if the cohort is not chosen randomly with respect to certain variables.
For example, if a study on the link between diet and heart disease only includes participants who are health-conscious and more likely to follow a healthier diet, the results may not apply to the broader population that includes people with a wider range of dietary habits. To mitigate selection bias, researchers must ensure that they select a randomized sample where every individual in the target population has an equal chance of being chosen.