Can estimate risk directly when follow-up starts at same time, little loss to follow- up, and no death from competing risks. Number at risk at baseline difficult to determine with staggered entry. Need to assume uninformative censoring in order for incidence rate/ hazard rate to solve problem of variable entry, loss to follow-up, and competing risks. C~i t is wrong b/c even if no loss to follow-up/competing risks, the # people at risk decreases. Cross-sectional studies can"t distinguish between duration and incidence. When there is homogeneity of a measure of effect on ratio scale, usually emm on difference scale, vice versa. Ar%: fxn of the disease among exposed attributed to exposure. Par%: excess or deficit risk which would occur in the population if the exposure were removed from population. Closed cohorts only need one common time scale for which the membership event is defined. Risks cannot be directly measured in an open cohort.