Table 1.

Notational framework for combining group- and individual-level data from a generic county, for the North Carolina low-birth-weight example. Upper-case letters denote information representing all individuals in the county; lower-case letters denote information on a sub-sample. Quantities in square brackets are not observed in an ecological study.

(a) Group-level outcome totals				(b) Group-level covariate totals				(c) Individual-level data

Race/ Sex	LBW = 0	LBW = 1			Race = 0	Race = 1		Race/ Sex	LBW = 0	LBW = 1
0/0	[N₀₀₀]	[N₁₀₀]	[M₀₀]	Sex = 0	[M₀₀]	[M₁₀]	M₊₀	0/0	n₀₀₀	n₁₀₀	m₀₀
1/0	[N₀₁₀]	[N₁₁₀]	[M₁₀]	Sex = 1	[M₀₁]	[M₁₁]	M₊₁	1/0	n₀₁₀	n₁₁₀	m₁₀
0/1	[N₀₀₁]	[N₁₀₁]	[M₀₁]		M₀₊	M₁₊	N	0/1	n₀₀₁	n₁₀₁	m₀₁
1/1	[N₀₁₁]	[N₁₁₁]	[M₁₁]					1/1	n₀₁₁	n₁₁₁	m₁₁
	N₀	N₁	N						n₀	n₁	n

LBW, low birth weight 0/1 = no/yes; Race 0/1 = white/non-white; Sex 0/1 = female/male.