[dba-Tech] Calculating Six Sigma - Please critique - Mathematicians where are you

Hello All,

I have to calculate a Six Sigma value and I have written a function for SQL
Server to calculate it.

>From what I have read, Six Sigma should really be looked up against a set of
values but I also think that it can be approximated using the formula at the
following link http://www.symphonytech.com/articles/sixsigma.htm.

The calculation is credited to Schmidt and Launsby 1997


Can I ask your assistance for the following

1) Are the calculations correct?
2) Would you rename the variables to more appropriate values that are still
readable by non-math experts

Here is the short version and then below I have also pasted my complete
function.  X, Y, and Z are the three static values in the Schmidt and
Launsby formula.


SET @X = 0.8406
SET @Y = 29.37
SET @Z = 2.221

SET @DPM=  1000000 / @OrderQty * @DefectQty

SET @NaturalLogOfDPM= log(@DPM)
SET @NaturalLogOfDPMTimesZ = @NaturalLogOfDPM * @Z
SET @CalculatedValueForSQR = @Y - @NaturalLogOfDPMTimesZ
SET @SQROfCalculatedValue = SQRT(@CalculatedValueForSQR)


SET @Result = @X + @SQROfCalculatedValue



Thanks
Mark

Here is the entire SQL Server udf



Create FUNCTION ufn_MB_CalcSixSigma
(
    @OrderQty NUMERIC(12,4),
    @DefectQty NUMERIC(12,4)
)
RETURNS NUMERIC(12,4)
AS
BEGIN

/*
The calculation is credited to Schmidt and Launsby 1997

*/


    DECLARE @DPM NUMERIC(12,4)

    DECLARE @X NUMERIC(12,4)
    DECLARE @Y NUMERIC(12,4)
    DECLARE @Z NUMERIC(12,4)

    DECLARE @NaturalLogOfDPM NUMERIC(12,4)
    DECLARE @NaturalLogOfDPMTimesZ NUMERIC(12,4)
    DECLARE @CalculatedValueForSQR NUMERIC(12,4)
    DECLARE @SQROfCalculatedValue NUMERIC(12,4)

    DECLARE @Result NUMERIC(12,4)


    SET @X = 0.8406
    SET @Y = 29.37
    SET @Z = 2.221

    IF @DefectQty <=0 OR @OrderQty < @DefectQty
        SET @Result = -99
    ELSE

        BEGIN

            SET @DPM=  1000000 / @OrderQty * @DefectQty

            SET @NaturalLogOfDPM= log(@DPM)
            SET @NaturalLogOfDPMTimesZ = @NaturalLogOfDPM * @Z
            SET @CalculatedValueForSQR = @Y - @NaturalLogOfDPMTimesZ
            SET @SQROfCalculatedValue = SQRT(@CalculatedValueForSQR)


            SET @Result = @X + @SQROfCalculatedValue

        End

    -- Return the result of the function
    RETURN @Result

END
GO