Forum Discussion

epicleo's avatar
epicleo
Frequent Visitor
9 years ago

Including 0 values when calculating STDEV

Hello Everyone,    I am attempting to calculate the 26 Week STDEV for weekly sales.  The Sales records are daily so I am using a Date table to aggregate them by week.   The problem I am coming in...
Need Help
v-huizhn-msft's avatar
v-huizhn-msft
Microsoft Employee
9 years ago

Hi epicleo,

As the GilbertQ said, if all they are null where sales are zero, you can change it by calculating a calculated column using the formula.

New Column=IF(ISBALNK(Table[column]),0,Table[column])

Then use the new column in your 26-W.STDEV formula, and check if it works fine.

Best Regards,
Angelia


  • epicleo's avatar
    epicleo
    Frequent Visitor
    9 years ago

    Sorry for the delay.  There is actually no value.  Let me walk you through a better example.  I am attempting to determine the most recent 6-Month STDEV for sales by product in order to determine variation.  

     

    The Sales Datais made up of daily transactions so it contains a   iContractsChargebacks[TransactionDate] and iContractsChargebacks[ChargebackUnits] (i.e. sales), but if there are no sales in a given month, then there will be no data for that month. 

     

    So, for example, on July 1st, sales for the past 6 months were the following:

     

    Jan 100

    Feb 125
    Apr 140
    May 125

    Jun 130

     

    March is missing because there were no sales.  So, when I calculate STDEV on the data set, it is calculating it over 5 periods, when in fact there were 6, just one happend to be zero.

     

    The code I am curretly using is:

     

    6M STDEV = CALCULATE(
    STDEVX.P(
    VALUES(DimDate[Calendar Month Number]),
    CALCULATE(SUM(iContractsChargebacks[ChargebackUnits]))
    ),
    FILTER(ALL(DimDate),
    DimDate[Month ID]<=MAX(DimDate[Month ID])-1 &&
    DimDate[Month ID]>=MAX(DimDate[Month ID])-6
    ))

     

    Instead of using date parameters in the code, I created a calculated column in the date table that gives each Month a unique ID, makes it easier for me.

     

    What I need the formula to do is calculate STDEV across the six month period.  If when pulling the monthly sales numbers, it only comes back with 3 periods, then it needs to assume the other 2 periods with a zero value.

     

    Thank you for your help with this!!

    • GilbertQ's avatar
      GilbertQ
      Super User
      9 years ago

      Hi epicleo


      What I would suggest doing is rather than to try and figure out when there is no data I would solve it by doing the following.

       

      I would left join from your DimDate table, to your Fact (Source Data) table. By doing a left join from the DimDate table it will then bring in all the dates. This will then allow your data to be blank when there is no data.

       

      Then based on your requirements you can then use the same measures and not it should show zero for the months where there is no data when you put in your Date column from your DimDate table.

      • epicleo's avatar
        epicleo
        Frequent Visitor
        9 years ago

        GilbertQ Thank you for the help; that would be a great work around except that I m dealing with more than 1000 SKUs so my query would produce a few million rows given we transact on a daily basis and are reporting on 3 to 5 year period.

         

        I thought about manaully calculating standard deviation instead of using the DAX STDEVX.P formula and found these 2 as a reference on how to do so, the first being closest to my need:

         

        https://community.powerbi.com/t5/Desktop/Problem-with-STDEV/td-p/19731

         

         

        https://stackoverflow.com/questions/42605377/calculating-the-standard-deviation-from-columns-of-values-and-frequencies-in-pow

         

        So, given my detail above, I attempted to make a go of it, but still am not getting the correct number.  My code is:

         

        STDEVX2 =
        var Averageprice=[6M Sales]
        var months=6
        return
        SQRT(
        DIVIDE(SUMX(
        FILTER(ALL(DimDate),
        DimDate[Month ID]<=(MAX(DimDate[Month ID])-1) &&
        DimDate[Month ID]>=(MAX(DimDate[Month ID])-6)
        ),
        (iContractsChargebacks[SumOfOrderQuantity]-Averageprice)^2),
        months
        )
        )

         

        I am determined to figure this out and I appreciate all your help with this.  Thank you in advance!!!