Table 1.

Individual vs. ecologic data

	Individual (interior cells)			Ecologic (margins)
	exposed	unexposed	sum	exposed	unexposed	sum
cases	16	12	28	?	?	28
noncases	24	48	72	?	?	72
total	40	60	100	40	60	100
risks	0.4	0.2		X	0.40
RD	0.2			Y	0.28
				n	100

Individual-level information on exposure and outcome are shown by the interior of a two-by-two table; they are summarized by the risks in the unexposed and exposed and the risk difference (RD). Ecologic studies possess only the margins of the table, summarized by the average exposure X, average risk Y and group size n.