Percentage mammographic density, age, and histological measures. Boxplots showing the associations of percentage density with age and histological measures. Median is shown by a horizontal line, mean by the '+' symbol, interquartile range by the columns, 1.5× the interquartile range by the whiskers, and outliers are shown separately. P values from linear regression, using continuous variables adjusted for age, were as follows: age, P = 0.04; total nuclear area, P < 0.001; epithelial nuclear area, P ≤ 0.001; nonepithelial nuclear area, P < 0.001; collagen, P < 0.001; glandular area, P < 0.001. Data from Li and coworkers .