Stdev() on data for row value not entire dataset

[nbuchholz1]

Hi All - I'm trying to use the stdev() function but am struggling to apply it to meet our needs. 
Here is a sample data that is similar to our data structure, we have a product, a week, and the total sales. 

I'm trying to identify the standard deviation of product sales across the all weeks excluding the current week for each product, lets assume current week is week 5. 
Using the following formula in my straight table:   stdev({<Week<={3}>}Sales), I can only get the stdev of all sales for that specific week range. 

Can anyone help me figure out how to return the standard deviation across weeks for each product? Thanks so much!

Product	Week	Sales
Product 1	Week 1	1000
Product 2	Week 1	2000
Product 3	Week 1	3000
Product 1	Week 2	1400
Product 2	Week 2	2001
Product 3	Week 2	4920
Product 1	Week 3	1892
Product 2	Week 3	3892
Product 3	Week 3	2378
Product 1	Week 4	9772
Product 2	Week 4	781
Product 3	Week 4	3922
Product 1	Week 5	3888
Product 2	Week 5	3789
Product 3	Week 5	1999

[dirk_fischer · Accepted solution]

Try using the Aggr function - you may not have the week in your table, then you get the Standard Deviation per product.

=Aggr({<WEEK={"<=3"}>}
    StDev({<WEEK={"<=3"}>}SALES),
          PRODUCT
         )