Why is my box plot not showing median line?

[Titir Bhattacharya]

Hi,

I just made a box plot with

graph box grade_15, over(shock_c) over(cohort) name(box_grade_shock_c,replace)nooutsides asy

and it produced plots (attached below) where the median line is not present for all but one of the boxes.

I would have shared example data but I tried the plot with the example data and it's not covering enough of the variation to produce all 4 boxes.

I would appreciate it if some of you can suggest the possible reasons behind this so that I may check for those issues in my data.

Please let me know if you would need more details.

Thanks,

Below is the distribution of the variable:

sum grade_15 if cohort==1,detail

                  grade attained at age 15
-------------------------------------------------------------
      Percentiles      Smallest
 1%            4              0
 5%            6              1
10%            7              2       Obs               1,822
25%            8              2       Sum of Wgt.       1,822

50%            9                      Mean           8.345225
                        Largest       Std. Dev.      1.393846
75%            9             11
90%           10             11       Variance       1.942808
95%           10             12       Skewness      -1.308763
99%           11             12       Kurtosis       6.291034

sum grade_15 if cohort==0,detail

                  grade attained at age 15
-------------------------------------------------------------
      Percentiles      Smallest
 1%            2              0
 5%            5              1
10%            6              1       Obs                 961
25%            7              1       Sum of Wgt.         961

50%            9                      Mean           8.150884
                        Largest       Std. Dev.      1.726968
75%            9             11
90%           10             11       Variance       2.982418
95%           10             11       Skewness      -1.465881
99%           10             12       Kurtosis       5.617447

[Rich Goldstein]

because you only show us 2 pieces of data and we really need to see all 4 that are shown in the box plot, I can't be sure, but my guess is that the median aligns with with the top or the bottom of the boxes - this is certainly true of the 2 summaries you show where, in each, the median is equal to the 75th percentile (which is the top of the boxes)

[Nick Cox]

I agree with Rich Goldstein. Indeed, box plots are in my view a poor choice for this kind of data: use histograms or quantile plots instead.