Figure 8-1 A, The normal gaussian distribution, with mean (average) and standard deviations (SDs) indicated. For many traits, the “normal” range is considered the mean ± 2 SD, as indicated by the shaded region. B, Distribution of systolic blood pressure in approximately 3300 men aged 40 to 45 (solid line) and approximately 2200 men aged 50 to 55 (dotted line). The mean and ± 2 SD are shown above double-headed arrows. See Sources & Acknowledgments.
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