Queries :: Comparing Pairs Of Records In A Query

I need to identify pairs of records by their ID and date fields, then compare their number values to get a text result.

I have a query that has the following fields

AuthInstanceID, ConsumerID, AuthNumberID, AuthStart, AuthEnd, PSRUnits, BSTUnits.

Sample data is below (PSRChange and BSTChange are fields I want to self populate based on the comparison I'm trying to do)

AuthInstanceID....ConsumerID....AuthNumberID....Au thStart....AuthEnd....PSRUnits....BSTUnits - PSRChange -BSTChange
1374006036.........356679..........20255102....... ....4/22/13.......7/21/13....0.............416~~~~-Loss~~~~~-None
-432536491.........356679...........20255102....... ...1/21/13.......4/21/13....104..........416~~~~-Loss~~~~~-Loss
-124970517.........356679...........20147863....... ...10/23/12.....1/20/13....208..........520~~~~-None~~~~~-Null
504564357...........469432..........20254788...... ....4/22/13.......7/21/13....0.............520~~~~-None~~~~~-None
282523535...........469432..........20254788...... ....1/21/13.......4/21/13....0.............520~~~~-None~~~~~-Gain
356661326...........469432..........20254788...... ....11/20/12......1/20/13....0.............416~~~~-None~~~~~-Null

What I need to do is compare these records in the following fashion:

Identify pairs of records where ConsumerID is the same AND AuthStart dates are successive. Then compare PSRUnits of more recent record (by AuthStart) with the previous record (by AuthStart) and determine the change of PSRUnits between them. If change is positive, "Gain" in field PSRChange, "Loss" for a loss, "None" for no change, and "Null" if there is an error due to no previous record to compare with. Do the same with BSTUnits/BSTChange

I've tried adding the following fields, but this did not work

PrevEnd: (Select Max(AuthEnd) from tblAuthorizations Where AuthEnd < Auths.[AuthStart])
PrevBST: DLookUp("[BSTUnits]","tblAuthorizations","AuthEnd=" & [PrevEnd] And "ConsumerID=" & [ConsumerID])