Final answer:
Mortality threatens external validity in observational studies because it introduces the possibility of a lurking variable that explains the correlation. To establish cause-and-effect relationships, researchers conduct experiments where they control all variables and randomize assignment to treatment groups.
Step-by-step explanation:
Mortality threatens external validity in observational studies because it introduces the possibility of a lurking variable that affects both variables of interest and explains the correlation. This means that the observed correlation may not be due to a cause-and-effect relationship between the variables, but rather due to the influence of an unmeasured variable.
An example of this confusion is the correlation-causation fallacy, which occurs when people mistakenly assume that a correlation between two variables implies a causal relationship. Just because two variables are correlated does not mean that one variable causes the other.
To mitigate the threats of mortality and lurking variables, researchers conduct experiments where they control all possible variables and randomize the assignment of subjects to treatment groups. This allows them to establish cause-and-effect relationships between variables.