That is, alternative plausible explanations for the outcome have been ruled out, and the only thing left to explain the results is the influence of the independent variable. Hopefully you will also have managed to design your study so that your results are generalisable. This means the results can be applied beyond your study to other populations, conditions and settings. The issue is whether such a population is representative of people who are NOT undergraduate psychology students. There are times when a lack of external validity is not a problem - you may not want or need to be able to generalise across people, conditions and settings.
Internal validity is however crucial to good research.
A study may have internal validity without external validity, but a study without internal validity will also have no external validity. Quantitative research for the behavioral sciences. This is a personal page published by the author. The ideas and information expressed on it have not been approved or authorised by Monash University either explicitly or impliedly. In no event shall Monash University be liable for any damages whatsoever resulting from any action arising in connection with the use of this information or its publication, including any action for infringement of copyright or defamation.