Hemoglobin recovery curves, including all donor returns, subdivided on the basis of donation frequency. Frequent donors (more than 15 for men, more than 10 for women) are shown in green, those of intermediate activity (6-15 for men, 6-10 for women) in red, and those of infrequent activity (5 or fewer for both sexes) in blue. The black line shows the population average. The upper panels show the distributions of the donor returns, and the lower panels show recovery (∆Hb) as a function of return interval (∆T in days). The 95% confidence intervals are shown as gray shading.