Which of the following is not a threat to validity?

A double-blind design is a methodological approach that minimizes bias and enhances the validity of a study's findings. In a double-blind study, neither the participants nor the researchers know which group participants belong to (experimental or control), reducing the potential for both participant and researcher bias. This design is particularly important in experimental research, as it helps ensure that the treatment effects can be attributed to the intervention rather than to external influences or expectations.

On the other hand, factors like measurement bias, selection bias, and issues related to sample size can threaten the validity of research findings. Measurement bias occurs when the tools used to collect data do not accurately measure the intended variables. Selection bias arises when the sample is not representative of the population, leading to skewed results. While increasing sample size can often enhance the reliability of a study, inappropriate handling of sample size can also introduce threats, but an increase typically is not a direct threat to validity when done correctly.

Therefore, the double-blind design stands out as a methodological strategy that actively works to protect the validity of a study, making it not a threat to validity.