External validity (or generalizability) is truth beyond a study. A study is external valid if the study conclusions represent the truth for the population to which the results will be applied because both the study population and the reader’s population are similar enough in important characteristics. The important characteristics are those that would be expected to have an impact on a study’s results if they were different (e.g., age, previous disease history, disease severity, nutritional status, co-morbidity, ...). Whether or not the study is generalizable to the population of interest to the reader is a question only the reader can answer. External validity can occur only if the study is first internally valid.
Truth within a study. A study is internally valid if the study conclusions represent the truth for the individuals studied because the results were not likely due to the effects of chance, bias, or confounding because the study design, execution, and analysis were correct. The statistical assessment of the effects of chance is meaningless if sufficient bias has occurred to invalidate the study. All studies are flawed to some degree. The crucial question that the reader must answer is whether or not these problems were great enough that the study results are more likely due to the flaws than the hypothesis under investigation.
In a study, the principle of keeping all things between groups similar except for the treatment of interest. This means that the same instrument is used to measure each individual in each group, the observers know the same things about all individuals in all groups, randomization is used to obtain a similar allocation of individuals to each group, the groups are followed at the same time.