How to create calc that uses data from same field different record?

[bena]

I was wondering how I could create a calculation that references data from a field but from a different record.

So if I had a field "weight", I would like to create a calculation that would show the Delta between the current amount and the previous records weight data.

So if weight were 10 in record 1 and 20 in record 2.. I need to understand how to tell FMP that I want to operate on different records.

The expression would look something like this:

Weight (current record) - Weight (Current record -1)=

How do I express this in FMP?

[comment]

See:

http://fmforums.com/forum/showpost.php?post/267325/