Table 6

Notation for a general two-by-two table

exposed

unexposed

sum

cases

p_im_i1

q_im_i0

p_im_i1+q_im_i0

noncases

(1-p_i)m_i1

(1-q_i)m_i0

(1-p_i)m_i1 + (1-q_i)m_i0

total

m_i1

m_i0

n_i = m_i1+m_i0

risks

p_i

q_i

RD

p_i-q_i

X_i

m_i1/n_i

Y_i

(p_im_i1+q_im_i0)/n_i

Let p_i and q_i be the risks of the exposed and unexposed in group i. Let m_i1 and m_i0 be the numbers of exposed and unexposed in group i. RD = risk difference; X_i and Y_i are the average exposure and average risk in group i; n_i = size of group i.

Webster Environmental Health 2007 6:17   doi:10.1186/1476-069X-6-17