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Internal validity dictates how an experimental design is structured and encompasses all of the steps of the scientific research method. Even if your results are great, sloppy and inconsistent design will compromise your integrity in the eyes of the scientific community. Internal validity and reliability are at the core of any experimental design. External validity is the process of examining the results and questioning whether there are any other possible causal relationships. Control groups and randomization will lessen external validity problems but no method can be completely successful. This is why the statistical proofs of a hypothesis called significant, not absolute truth. Any scientific research design only puts forward a possible cause for the studied effect.
A key concept relevant to a discussion of research methodology is that of validity. When an individual asks, Is this study valid?, they are questioning the validity of at least one aspect of the study. There are four types of validity that can be discussed in relation to research and statistics.