If the PLOT option is specified on the PROC UNIVARIATE statement, BY-group processing is used, and if one or more BY groups have either constant or all missing values for the analysis variable then the side-by-side box plots will be incorrect. When only the first BY group has constant or all missing values, the box plots themselves are correct but the labels will all be shifted over one position to the left and the last BY group label is used twice. That is, the value of the second BY variable(s) appears under the first box plot, the value of the third BY variable(s) appears under the second box plot, and so on. The last two boxes are both labeled with the value of the last BY variable(s). If any BY group beyond the first contains constant or all missing values for the analysis variable, then one or more of the b ox plots will be incorrect in addition to the labels being incorrect. The problem is always identifiable by that fact that two of the box plots will have the same label.

To circumvent this problem, omit the side-by-side box plots with the statement

ODS LISTING EXCLUDE SSPLOTS;

and instead use either the BOXCHART statement in PROC SHEWHART or PROC GPLOT with the option INTERPOL=BOX on a SYMBOL statement or PROC BOXPLOT. Note that in the default plots produced by PROC SHEWHART, PROC GPLOT and PROC BOXPLOT, the whiskers are drawn to the extreme values in each BY group. Refer to the Online Documentation for options on alternate specifications.