Solveig is testing whether a new experimental drug she designed can improve immune functioning. She recruits participants for a clinical trial and randomly assigns them to either a treatment group, in which they receive the drug, or a control group, in which they receive a placebo. After several weeks, she measures the differences between the two groups' immune functioning via blood test. Solveig announces that the treatment group had better immune functioning after the trial and claims that her drug was effective. Though the immune functioning between the two groups was different, that difference was not statistically significant. Assuming that Solveig's drug is effective, which type of validity might be insufficient in this study? internal external construct statistical