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Find difference between two consecutive records in same field

Question asked by wdrspens on Jan 1, 2018
Latest reply on Jan 1, 2018 by alquimby

I need to find the amount by which a field has increased since the last measurement.

How do I achieve that? 

In essence I need to subtract myfield(ID-1) from my present field, myfield(ID) immediately after entering the myfield(ID) value.  Myfield(ID) will always be greater than the previous myfield value. 

But what is the calculation to achieve this and how do I ensure it will always work, even when refreshing the table?

I have tried using GetNthRecord, but I frequently get a "?" in the result field.  I have also tried LookUp but that, of course, works on a different but related table.  I am not trying TO but have got bogged down with parent and sibling!

Any suggestions?  I am a reasonably experienced Microsoft Access designer, but a complete newbie with Filemaker.

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