Variable on which people are observed to differ. Varied on dependent measures because exposed to different levels of independent variable. Variable that explains or causes range of scores in dependent variable. Age, but not really because its value cannot be controlled or manipulated therefore use range of statistical techniques = not true. Iv = can"t be randomly assigned to age group = never state with certainty that aging caused people to receive certain scores on dv. = only whether different age groups varied in their responses to different levels of iv or correlation (ex. Memory test for old and young = can"t determine if age caused differences) therefore study of aging = methodological challenge. Designs lack ability to determine cause and effect but understanding differences between age group still good. Same people followed repeatedly over time and compared at different ages to determine whether people changed over time as result of aging.