Fig. 2. Number of subjects required to detect the association between candidate polymorphisms and increased risk of chronic pain versus  the number of independent polymorphisms tested. The injury or disease is assumed to produce an incidence of chronic pain = 20% in patients unexposed to the (candidate) minor allele. The three curves  in each panel correspond to relative risks (RRs) of 1.5, 2.0, and 2.5 conferred by exposure to at least one copy of the minor allele in a dominant model. The four panels  show population frequency of the minor allele as 5% (top left ), 10% (top right ), 20% (bottom left ), and 30% (lower left ). n^E= number exposed; n^U= number unexposed.