Clinical Trials

Clinical Trials: A Practical Guide ■❚❙❘Figure 6. A stem and leaf plot to show the baseline age (years) for patients who initiated ritonavir-boostedsaquinavir in the MaxCmin 1study.1 | 92 | 457893 | 00000111112222222222333333333344444445555555566666777777888888999999994 | 1111112222233333344444555566666677888995 | 001111122223355666677778896 | 0011237 | 1Note. The number on the left side of ‘|’ stands for the number of years (as a multiple of 10) and the number onthe right side of ‘|’ represents individual years. For example,‘1 | 9’ corresponds to a patient who is 19 years old.quartile and the top to the upper quartile, while the whiskers terminate at themost extreme values in the dataset (including outliers).If the data are not skewed then the plot will be perfectly symmetrical – the medianwill lie in the center of the box and both whiskers will extend for the same distanceon either side. If the data are positively skewed then the median will be displacedto the bottom of the box, and if they are negatively skewed then the median willlie towards the top. There will also be an influence on the length of the whiskers.For data that contain wide variation between observations it is more advisable touse a truncated box plot (Figure 7). In a truncated box plot, the whiskers extendout a specific statistical distance from the box (to the 5th and 95th percentiles, say)or to a point no further from the box than the IQR; all other observations (theoutliers) are presented as dots beyond the whiskers. Different groups can bevisually compared by aligning the box plots for each dataset, as long as they havebeen produced on the same scale. When more than one group is compared thenthe relative position of the horizontal line indicates the difference betweenmedians. The relative height of the boxes indicates the difference in variation, andthe relative overall height of the plots shows the difference between the ranges.415