Q172084: PRB: CALCULATE STD–Standard Deviation–Gives Numeric Overflow

Article: Q172084
Product(s): Microsoft FoxPro
Version(s): MACINTOSH:3.0; WINDOWS:3.0,3.0b,5.0,5.0a
Operating System(s): 
Keyword(s): kbcode kbvfp
Last Modified: 02-MAY-2001

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The information in this article applies to:

- Microsoft Visual FoxPro for Windows, versions 3.0, 3.0b, 5.0, 5.0a 
- Microsoft Visual FoxPro for Macintosh, Professional Edition, version 3.0 
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SYMPTOMS
========

When calculating the standard deviation using the CALCULATE STD() function,
FoxPro can return results with a numeric overflow on certain numerical values
over various ranges of records.

RESOLUTION
==========

Use a custom routine that calculates the standard deviation. The following
program accepts two parameters for the table and the field. Then it calculates
the various sums and products of the fields before computing the standard
deviation. Create a program called std and enter the following code:

     * Call this program by ?std("TableName","FieldName") or
     * by assigning the result to a variable, y= std("TableName","FieldName")
     *
        PARAMETER mtable mfield
        SELECT SUM(&mfield^2) FROM (mtable) INTO ARRAY yy
        SELECT SUM(&mfield)^2 FROM (mtable) INTO ARRAY zz
        SELECT COUNT(&mfield) FROM (mtable) INTO ARRAY xx
        stdev=((xx*yy-zz)/xx^2)^.5
        RETURN ROUND(stdev,12)

If a table named values contains the field num, enter this command in the Command
Window:

     ?std("values","num")

STATUS
======

Microsoft is researching this problem and will post new information here in the
Microsoft Knowledge Base as it becomes available.

MORE INFORMATION
================

The numeric overflow is generally observed when a table contains the same
repeated real values over a broad range of records. For example, if a table
contains 10 records and each record's value is 0.031, the CALCULATE STD() will
produce an overflow. This can vary depending on the value and the number of
records.

One way to detect that there was a numerical overflow is to display the result of
the CALCULATE STD() to the desktop or in a WAIT WINDOW. If SET TALK is ON, the
result will be displayed on the desktop automatically. If there is an overflow,
all asterisks will be displayed. Also, if the result of the CALCULATE STD() is
appended or replaced to a numerical field in a table, the following error may
occur:

  Numeric overflow. Data was lost.

Steps to Reproduce Behavior
---------------------------

Enter the following code in a program and run it:

     CLEAR
     SET SAFETY OFF
     SET DECIMALS TO 12
     CREATE TABLE stdresult (num N(15,8)) &&Table for result of calculation
     APPEND BLANK
     CREATE TABLE stdtest (num N(15,8))   &&Table of values for calculation
     FOR i = 1 TO 10
        INSERT INTO stdtest(num) VALUES (0.031)
     ENDFOR

     FOR i = 1 TO RECCOUNT()
        ?ALLTRIM(STR(i))+" "
        SET TALK OFF
        CALCULATE STD(num) FOR RECNO()<i TO nresult && Comment this line &
                                                    && the next line once
                                                    && you uncomment the
                                                    && commented lines below.

        ??nresult

        *nresult=custstd('stdresult','num') && Uncomment this line & the next
                                            && line to compute & display the
                                            && standard deviation with a
                                            && user-defined function.

        *?nresult
        SELECT stdresult
        REPLACE num WITH nresult   &&Overflow error on 10th iteration
        SELECT stdtest
     ENDFOR

     SET SAFETY ON
     SET TALK ON
     SET DECIMALS TO 2

     PROCEDURE custstd
     PARAMETER mtable mfield
     SELECT SUM(&mfield^2) FROM (mtable) INTO ARRAY yy
     SELECT SUM(&mfield)^2 FROM (mtable) INTO ARRAY zz
     SELECT COUNT(&mfield) FROM (mtable) INTO ARRAY xx
     stdev=((xx*yy-zz)/xx^2)^.5
     RETURN ROUND(stdev,12)

After running this code, the first nine iterations should be all zeroes. The
calculation will be all asterisks and will produce the overflow error message on
the REPLACE command.
(c) Microsoft Corporation 1994, All Rights Reserved.
Contributions by Dean Christopher, Microsoft Corporation



Additional query words:

======================================================================
Keywords          : kbcode kbvfp 
Technology        : kbHWMAC kbOSMAC kbVFPsearch kbAudDeveloper kbVFP300Mac kbVFP300 kbVFP300b kbVFP500 kbVFP500a
Version           : MACINTOSH:3.0; WINDOWS:3.0,3.0b,5.0,5.0a
Issue type        : kbprb

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