Whether you're a student, teacher, homeschooler, principal, adult returning to the classroom after 20 years, or a friendly alien trying to get a leg up in earthly biology - Khan Academy's personalized learning library is available to you, for free. Or, if you're feeling particularly adventurous, learn how fire-stick farming changes the landscape of Australia. Get a head start on next semester's geometry. Confounding variable changes in dependent variable may be due to existence of or variations in a third variable Temporal confounds time related confounding variables Vehicular control when you're testing a drug that's, say, injected intravenously dissolved in saline, then the vehicular control condition is just injecting the saline (the vehicle for the drug) by itself, to make sure that it's the drug, not the vehicle, that's having an effect on the subject.Spend an afternoon brushing up on statistics. To protect external validity, sample must be completely random, and all situational variables must be tightly controlled construct validity Construct validity is 'the degree to which a test measures what it claims, or purports, to be measuring.' Regression to the mean a statistical phenomenon where, by random chance, a dependent variable of either the program or nonprogram group is at an extreme end of the distribution (either high or low), and subsequent measurements are more in line with the central tendency.

External validity Whether results of the study can be generalized to other situations and other people.

Confounding factors often impact the internal validity of an experiment. Internal validity extent to which a causal conclusion based on a study is warranted.

Clinical trials highly controlled interventional studies Randomized controlled trial people studied randomly given one of treatments under study, used to test efficacy/side effects of medical interventions like drugs. comparing people with the disease with those who don't but are otherwise similar. Case-control study observational study where 2 groups differing in outcome are identified and compared to find a causal factor.

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