Internal validity is defined as:

Internal validity refers to the degree to which the outcomes of a study can legitimately be attributed to the intervention being tested, rather than to other extraneous factors or variables. This concept is critical in research design because it helps to ensure that any observed effects in the study are a result of the treatment or intervention applied, as opposed to other influences that could skew results.

For example, if a study investigating a new prevention program shows significant positive outcomes, high internal validity would mean that we can confidently say those outcomes are due to the program itself and not to confounding variables, such as participant characteristics or external environmental factors. A strong internal validity enhances the credibility and reliability of the study's findings.

In contrast, the other options address different aspects of research but do not define internal validity accurately. The applicability of study outcomes to different populations pertains to external validity, while the likelihood of repeated success in future studies touches on the reliability or generalizability of the findings. Lastly, while the accuracy of data collected is important, it is more related to the methodology and data collection process rather than the specific concept of internal validity.