The PMHNP submitted an original research paper to a journal for publication, but it was rejected due to lack of internal validity. Which of the following most accurately defines internal validity?

Choice A reason: This describes probability, not internal validity. It is relevant to statistical inference but does not address the causal relationship within a study. Choice B reason: This refers to statistical dispersion or variability, such as standard deviation. It is a descriptive statistic and unrelated to internal validity. Choice C reason: Internal validity refers to the degree to which a study establishes a causal relationship between the independent and dependent variables. It ensures that observed effects are due to the intervention and not confounding factors, making this the correct definition. Choice D reason: This describes external validity, which concerns the generalizability of findings to other populations or settings. It is distinct from internal validity.