In fact, the 25 th percentile of the weights in men is approximately 180 pounds and equal to the 75 th percentile in women. Specifically, 25% of the men weigh 180 or less as compared to 75% of the women. There are many outliers at the high end of the distribution among both men and women. There are two outlying low values among men.įigure 13 - Side-by-Side Box-Whisker Plots of Weights in Men and Women in the Framingham Offspring Studyīecause men are generally taller than women (see Figure 14 below), it is not surprising that men have higher weights than women.įigure 14 - Side-by-Side Box-Whisker Plots of Heights in Men and Women in the Framingham Offspring Studyīecause men are taller, a more appropriate comparison is of body mass index, see Figure 15 below.įigure 15 - Side-by-Side Box-Whisker Plots of Body Mass Index in Men and Women in the Framingham Offspring Study Figure 13 below shows side-by-side box-whisker plots of the distributions of weights, in pounds, for men and women in the Framingham Offspring Study. The figure clearly shows a shift in the distributions with men having much higher weights. The "whiskers" of the plot (boldfaced horizontal brackets) are the limits we determined for detecting outliers (47.5 and 99.5).įigure 12 - Box-Whisker Plot of Diastolic Blood Pressures with Full Sample (n=3,539) of Participantsīox-whisker plots are very useful for comparing distributions. At the high end of the distribution, there are 12 values that are considered outliers (i.e., values above 99.5 which was the upper limit for determining outliers). At the low end of the distribution, there are 5 values that are considered outliers (i.e., values below 47.5 which was the lower limit for determining outliers). In Figure 12 the outliers are displayed as horizontal lines at the top and bottom of the distribution. We determined that there were no outliers in the distribution of diastolic blood pressures in the subsample of n=10 participants who attended the seventh examination of the Framingham Offspring Study.įigure 11 - Box-Whisker Plot of Diastolic Blood Pressures in Subsample of n=10.įigure 12 is a box-whisker plot of the diastolic blood pressures measured in the full sample (n=3,539) of participants. Recall that in the full sample we determined that there were outliers both at the low and the high end (See Table 16). A box-whisker plot is meant to convey the distribution of a variable at a quick glance. The median is the 50 th percentile, the third quartile is the 75 th percentile and the maximum is the 100 th percentile (i.e., 100% of the values are at or below it).Ī box-whisker plot is a graphical display of these percentiles. Figure 11 is a box-whisker plot of the diastolic blood pressures measured in the subsample of n=10 participants described above in Table 14. The horizontal lines represent (from the top) the maximum, the third quartile, the median (also indicated by the dot), the first quartile and the minimum. The shaded box represents the middle 50% of the distribution (between the first and third quartiles). A specific quantile or percentile is a value in the data set that holds a specific percentage of the values at or below it. The first quartile, for example, is the 25 th percentile meaning that it holds 25% of the values at or below it. These are sometimes referred to as quantiles or percentiles of the distribution. Box-Whisker Plots for Continuous VariablesĪ popular graphical display for a continuous variable is a box-whisker plot. Outliers or extreme values can also be assessed graphically with box-whisker plots. For the subsample of n=10 Framingham participants who we considered previously we computed the following summary statistics on diastolic blood pressures:
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