Hey @SaiKrishna3! It looks like you're trying to apply an error percentage, but you'd like to do it by group. The best way to do this is to calculate your error percentage as a standard, unaggregated measure:
('Actual'n - 'Predict'n) / 'Actual'n
You can then create a box plot for each by-group with the Category or Lattice data roles. Box plots do not support aggregated measures. Aggregating data and applying it to a box plot will produce a single point, and aggregating a box plot to a specific by-group will create a set of points for each group but they will not necessarily be helpful for interpreting your percent error as a single box plot. In that case, you're better off using a bar chart grouped by a category to get the average error of each group.
If you want to focus only on a specific by-group, it is best to add a filter / control that lets you select the specific group of interest.
My personal recommendation is to use a histogram for interpreting error for normality assumptions and reviewing extremes.
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