The significance level measures the proportion of the time an inference about a population will be correct in the long run.

The significance level, also known as alpha, is the threshold chosen by the researcher to test the null hypothesis. It represents the probability of rejecting the null hypothesis when it is true, and is typically set at 0.05 (5%). It does not measure the proportion of times an inference about a population will be correct in the long run. This concept is related to the concept of confidence level, which measures the proportion of times the true population parameter will fall within the confidence interval calculated from a sample in the long run.