Help with Formula Driven by value in a Drop down list

[ZEROCOOL UK]

Hi Folks

I need some help with what on the face of it seems like a very simple calcuation but in reality is proving difficult for me to solve.

I've uploaded a basic example of the equation I am trying to solve.

The Spreadsheet has 2 tabs, Main and Data.

On main there in Cell B2 is a basic drop list with the current week.
When I select the week I'd like the sheet to look up the appropriate week from the Data Tab and calculate the percentage difference between the current and previous week.

What I am struggling with is getting the calculation to be driven by the drop list.

Can you help please?

[63falcondude]

This will return the current week:

=INDEX(Data!B5:K5,MATCH(B2,Data!B4:K4,0))

This will return the previous week:

=INDEX(Data!B5:K5,MATCH(B2,Data!B4:K4,0)-1)

[63falcondude]

Thanks for the rep!