How Can I Get The Lag Function to "Reset" With Each New Batch?

[#1]

Hello, 

I have the following data and I want to use the Lag function to calculate the difference between each result and the result prior to it (the data table is also attached to this message).

I am using JMP 16.

Currently I'm using the following formula in the "Difference" column:

What I can't figure out is how to get the calculation to "reset" once it gets to Batch B. I realize that even with my current formula, only the difference value in row 16 is incorrect, but still I would know to setup the formula so that it works properly within batches.

Does anyone have any suggestions?

Jeff

[#2 · Accepted solution]

change your formula to

If( :"Time Point (months)"n == 0,
	.,
	:Result - Lag( :Result )
)

[#3]

Thank you for the quick replies @txnelson @jthi 

Much appreciated!!

[#4]

If you need such grouped lag operations frequently, you might have a look at 
https://marketplace.jmp.com/appdetails/Normalization+GUI in the Marketplace.

It implemented the N-1 in combination with grouping based on  @jthi 's approach:

This takes the pain to remember the exact syntax : )

In addition, the Platform handles Excluded rows automatically.

[#5]

Dif() and Lag() formula functions by column group (byVar argument) - great wish by @FN 

-> In JMP19, lag() and dif() can be calculated via  Col At () - which provides a grouping option.

[#6]

You can also utilize Col Moving Average to get the previous value (Data preparation- substract column data by other column )

:Result- (Col Moving Average(:Result, 1, 1, 0, :Lot Number) * 2 - :Result)

but in your case using something like Jim suggested is most likely better. Also there is Dif function you can use instead of Lag

If( :"Time Point (months)"n == 0,
	.,
	Dif(:Result, 1)
)