To Create a Variable which Subtracts the Current Row Value from Previous Row Value of teh same column, please follow the below steps.

1) Create a Variable “Previous_Value” and add $V{Current_Value} as the expression. This will Give a Invalid error as the Current_Value field is not created yet.

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2) Create Current_Value Field and ass your column as expression.

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3) Now Create the Difference Variable with subtract expression. (Here I will Create integer Variable and Convert My Prev and Curre Variable to integer.)

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4) Now when we add the Diff Field into the report, the Report will display the difference from Currnet row value to the previous row value.

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