This validity is concerned with the relationship between the treatment and the outcome. We want to make sure that there is a statistical relationship, i.e. with a given significance. Threats to conclusion validity are concerned with issues that affect the ability to draw the correct conclusion about relations between the treatment and the outcome of an experiment. These issues include, for example, choice of statistical tests, choice of sample sizes, care taken in the implementation and measurement of an experiment. Examples: Low statistical power, Violated assumption of statistical tests, Fishing and the error rate, Reliability of measures, Reliability of treatment implementation, Random irrelevancies in experimental setting, Random heterogeneity of respondents.

Wohlin, C., Runeson, P., Höst, M., Ohlsson, M., Regnell, B., & Wesslén, A. (2000). Introduction to Experimentation in Software Engineering. [-]