Return Values in of Cells in a Column where Two Other Values in Corr. Rows Match Criteria

Hi and welcome to the forum

Without being able to see what that formula is actually doing and what it's working with, it's kinda hard to offer many firm suggestions.

However, I would say that, as you are using 2010, you take a look at the iferror() function to simplify the formula a bit. So you can shorten...
IF(ISERROR(INDEX('Master'!$B$2:$B$20000,MATCH('Usage Staging'!$R2&'Usage Staging'!$O2,'Master'!$A$2:$A$20000&'Master'!$H$2:$H$20000,0))),"No Exact Member ID & State Pair Match",INDEX('Master'!$B$2:$B$20000,MATCH('Usage Staging'!$R2&'Usage Staging'!$O2,'Master'!$A$2:$A$20000&'Master'!$H$2:$H$20000,0)))
to...
=IFERROR(INDEX(Master!$B$2:$B$20000,MATCH('Usage Staging'!$R2&'Usage Staging'!$O2,Master!$A$2:$A$20000&Master!$H$2:$H$20000,0)),"No Exact Member ID & State Pair Match")

Also, to further simplify things, consider adding helper columns to combine...'Usage Staging'!$R2&'Usage Staging'!$O2 and Master!$A$2:$A$20000&Master!$H$2:$H$20000...so that you only need to refer to 1 cell and can also do away with the array function.

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