![]() ![]() 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. When the greater values are more spread out, the data are skewed to the right.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: Skewed used to describe data that is not symmetrical when the right side of a graph looks “chopped off” compared the left side, we say it is “skewed to the left.” When the left side of the graph looks “chopped off” compared to the right side, we say the data is “skewed to the right.” Alternatively: when the lower values of the data are more spread out, we say the data are skewed to the left.
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