How is the validity of an experimental design determined?

The validity of an experimental design is fundamentally determined by whether it accurately measures what it is intended to measure. This aspect of validity, often referred to as construct validity, ensures that the outcomes of the experiment truly reflect the phenomena being investigated. For example, if a study aims to assess the effectiveness of a new educational intervention on student learning outcomes, the measures used must closely align with learning outcomes (e.g., test scores, retention rates) rather than unrelated factors.

While including a control group is important for evaluating causality and ruling out confounding variables, it does not, in itself, guarantee that the experiment is valid regarding the constructs it intends to measure. Similarly, the duration of the experiment and the sample size can influence the reliability and generalizability of the results, but they do not directly address whether the measurements taken are meaningful and relevant to the hypothesis in question. Therefore, the key factor in establishing validity lies in the alignment between measurement tools and the concepts being investigated.