I have needed to describe validity recently and found it useful to paraphrase some of this statistics blog post: http://andrewgelman.com/2015/04/28/whats-important-thing-statistics-thats-not-textbooks/
I’ve been working a lot on air pollution modelling recently, where ‘validation’ is used to assess how well the modelled pollution predicted values represent the actual pollution observed. I tend to think of validity as formalised in statistical terms, i.e. as correlations between different measurements of the same thing, or between measurement and ‘truth’, and statistics are used for assessing and calibrating measurements.
I am guessing that when applied to the validity behind a research proposal, the issue might be whether the measurement is suitable for addressing the issue the researcher (and research question) is interested in, and therefore supporting the researchers to make valid inferences from the outcome of statistical methods. I have heard lots of anecdotes from statisticians in which when they are asked to help with analysing data, their advice was essentially that in order for a valid analysis that addresses the research question one would rather need to have collected different measurements.
