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When you plot a dataset with BoxWhiskerChart passing over the box in the plot with your mouse pointer shows some specifics about the dataset (min and max values, 1/4, 1/2, and 3/4 quartiles). However, if you use Quartiles or Median to calculate those values, they do not correspond. The values displayed by BoxWhiskerChart are different. Does anyone have an explanation or is it simply a bug? Is it system specific (here 8.0.1 on OSX 10.6.8)?

data = RandomVariate[NormalDistribution[0, 1], 20]
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What happens if you try with an odd number of elements? If I remember well, the Median[]definition changes for lists of odd and even lengths. – Dr. belisarius Jun 16 '12 at 17:31
@belisarius Good point. With an odd number of elements the median is correct, however, the 1/4 and 3/4 quartiles are still wrong. – VLC Jun 16 '12 at 17:35
@VLC: that would be "different", not "wrong". Mathematica allows for different parameters for Quantile[], as I have indicated in my answer. – J. M. Jun 16 '12 at 17:46
@J.M. you're right (or "different"). I wrote my comment just a sec before your answer appeared. – VLC Jun 16 '12 at 17:48
up vote 12 down vote accepted

What BoxWhiskerChart[] is actually using for the quartiles (and the median as well) is Quantile[data, {1/4, 1/2, 3/4}], or more explicitly, Quantile[data, {1/4, 1/2, 3/4}, {{0, 0}, {1, 0}}] (what the docs calls the "inverse empirical CDF" parameters).

Quartiles[data] (and thus also Median[]), on the other hand, is equivalent to Quantile[data, {1/4, 1/2, 3/4}, {{1/2, 0}, {0, 1}}] ("linear interpolation" parameters); these two different ways to specify the quantiles will almost always give different results for a particular data set.

Unfortunately, there does not seem to be an easy way to let BoxWhiskerChart[] know what parameters you prefer to use for Quantile[].

Brett's answer displays the (not-too-well documented) "BoxRange" suboption for Method; as I mentioned in a comment, the compact way to use that along with whatever quantile definition you prefer is BoxWhiskerChart[data, Method -> {"BoxRange" -> (Quantile[#, Range[0, 1, 1/4], {{1/2, 0}, {0, 1}}] &)}], and you can replace {{1/2, 0}, {0, 1}} with your desired parameter set.

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With the LabelingFunction Option you could at least show the desired values in the tooltip box – Dr. belisarius Jun 16 '12 at 17:40

There's an Method option, "BoxRange" that can be used to control the calculation of the box parameters:

BoxWhiskerChart[data, GridLines -> {None, Quartiles[data]},
   Method -> {"BoxRange" -> (Flatten[{Min[#], Quartiles[#], Max[#]}] &)}]

box-whisker chart

The first example under Properties & Relations that shows how the various properties of the chart are calculated, using a concrete dataset.

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Method -> {"BoxRange" -> (Quantile[#, Range[0, 1, 1/4], {{1/2, 0}, {0, 1}}] &)} is a bit more compact... – J. M. Jun 17 '12 at 4:49
@brett-champion, Do you know what is happening to the "Outliers" when you change the "BoxRange" method? See this post to see, some outliers are being removed when we use a custom box range. – JasonB Feb 24 at 11:14

By using J. M.'s answer, this will provide you with a "corrected" tooltip:

label[data_, index_, label_] := 
  Grid[{{Style["max", Bold], 1},           {Style["75%", Bold], #1[[2]]}, 
        {Style["median", Bold], #1[[3]]},  {Style["25%", Bold], #1[[4]]}, 
        {Style["min", Bold], #1[[5]]}}, 
     Dividers -> {{#, #}, {#, #}} &@{Directive[GrayLevel[0.3]], Directive[GrayLevel[0.3]]}, 
     Alignment -> {{Center, ".", {Left}}}, Frame -> GrayLevel[0], 
     BaseStyle -> Directive[AbsoluteThickness[1], Dashing[{}]]] &@
   Join[{Max@data}, Reverse@Quartiles[data], {Min@data}];

data = RandomVariate[NormalDistribution[0, 1], 20]; 

 LabelingFunction -> (Placed[label[##], Tooltip] &), 
 ChartLabels -> Placed[Range[2005, 2009], None]]

box-whisker chart with modified tooltips

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I'd replace Join[{Max@data}, Reverse@Quartiles[data], {Min@data}] with Quantile[data, Range[1, 0, -1/4], {{1/2, 0}, {0, 1}}] myself... – J. M. Jun 17 '12 at 4:43

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