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Hazard ratios for mortality based on the second classification system. Number of months with hemoglobin <11 g/dl and first or second half of the 6-mo exposure period. For example, 2-F represents 2 mo with hemoglobin <11 g/dl during the first 3 mo; 4-F-S represents 4 mo with hemoglobin <11 during both the first and the second 3-mo periods. I-I, consistently intermediate (11 to 12.5 g/dl), represents the reference group.
