What is the difference between external and internal validity of research study design? This job gives an example to understand these terms

Validity in research study design usually refers to the tools used to measure aspects of the research question, for example a questionnaire is used in survey design and is said to have 'internal validity' if the questions are precise enough to get answers which address the aims of the study. For example if you want to know about body mass you need precise height measurements and precise weights for the results to have internal validity.

'External validity' is satisfied if these same questions are generalizable or transferable to other studies with similar aims but which deal with a different population, for example children not adults, or a different place, e'g' America or Africa or a different time- i.e. in the summer or in the winter. If the questionnaire has 'external validity' it doesn't matter, who, where or when the questions are asked they will meet the aims of the study.