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Expression Builder problem with stddev function

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  • Expression Builder problem with stddev function

    I have two numeric fields in a .dbf table named,
    Result_1 and Result_2. In addition I have one
    more numeric field SD_1. Using the "Field Types" tab
    under Field Rules, I am trying to calculate the standard
    deviation of the Result_1 and Result_2 fields using the
    function stddev in expression builder but I cant seem to
    construct the statement correctly. I am assuming that I
    dont need the GRP->Group or GRP-> Sub-Group parameters because
    the calc is on only two fields in the current table but I still
    cant get the parameters right. Has anyone else struggled with
    this simple problem?

  • #2
    Re: Expression Builder problem with stddev function

    I'm not a statistician but it sounds like you're trying to define a calculated field for each record. But doesn't the std deviation function require a range of records? What does it mean to do this calc separately for each record using two field values each time?


    • #3
      Re: Expression Builder problem with stddev function

      Compee is trying to use the standard deviation function across two records, Result_1 and Result_2, not each record individually. This is a common practice in Laboratories where some analytical methods are performed in duplicate on each sample. The Standard deviation is used to measure the spread between the two results.


      • #4
        Re: Expression Builder problem with stddev function

        David, this is what I thought as well, but then why is he using "Field Rules" ? ... and doesn't his post imply that Result_1 and Result_2 are fields in each record?

        Wouldn't you think a better approach would be to put a calculated display field on the form, rather than trying to store the computed deviation result in each record?

        I'm obviously missing something here, so I'll shut up.

        -- tom


        • #5
          Re: Expression Builder problem with stddev function

          Thanks Tom,
          I use "Field Rules" because after I enter Result1 and result2, It automatic calculate standard deviation(SD) and put the SD value into the field Standard at the same record.
          Now I know how to solve that problem by create myfunction to calculate SD. I just do not understand the built in function "stddev()" how to apply to use
          I create function like this

          Function mysd as N ( Value1 as n,Value2 as n)
          dim mean as n
          dim sum as n
          mean = (value1 + value2)/2
          sum = (value1-mean)^2 + (value2-mean)^2
          mysd = sqrt(sum)
          end function

          and use in the Field Rule by check calculated field and insert the mysd function it work fine
          Last edited by compee; 09-04-2013, 01:12 PM.


          • #6
            Re: Expression Builder problem with stddev function

            Compee, I think the stddev() function in Alpha assumes you will be computing the deviation in field values ranging over several records, not the deviation in field values in the same record.


            • #7
              Re: Expression Builder problem with stddev function

              Thank you Tom, your answer clears up my problem. I didn't realize the limitation you described. That explains why we could not get the function to work with our table structure.

              David and Compee