Validity can be either:


Internal validity is a judgement on how well the study has been done and whether or not you accept the results from the study.

A perfectly conducted double-blind randomised controlled trial with no bias, confounding problems, or competing interests, has much more internal validity than an unblinded, very biased study funded by a drug company.


External validity means how well the results can be applied to other areas.

You should be a little sceptical of results from a study conducted anywhere but in an emergency department. Our patients are unlike anyone else’s!

Ask the following questions:

  • Is this study internally valid – do you accept the findings from the study?
  • Is this study externally valid – can you apply this to your own patients?
  • How much does this cost?

‘Validity’ means ‘how close to the truth is the result of this study?’

This is different from ‘reliability’, which means ‘if I was to repeat this study again, how likely is it that I would I find the same result?’ It is possible to have very reliable results that are invalid.