This document discusses key concepts in experimental design and variables. It defines an independent variable as one that is systematically manipulated by the researcher, and a dependent variable as one that is observed and measured to see the effects of changes in the independent variable. Extraneous variables are any other variables besides the independent variable that could influence the dependent variable. Experimental designs like independent groups, matched pairs, and repeated measures aim to minimize the effects of extraneous variables. Placebo and experimenter effects are also discussed, and double-blind procedures are proposed to control for these.