Random assignment of subjects to experimental and control groups has which of these effects?

ANS: C

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A Randomization has no effect on mortality (dropout rate).
B Randomization minimizes pretreatment variance between groups, rather than maximizing variance. This helps to ensure that posttreatment differences are related to the independent (treatment) variable alone.
C Random assignment to groups helps to ensure that other (extraneous) variables that could affect changes in the dependent variable are equally distributed within groups, thus reducing systematic bias.
D Randomization reduces selection bias, rather than increasing it.