Re: [R] SAS or R software

From: Ted Harding <Ted.Harding_at_nessie.mcc.ac.uk>
Date: Sun 19 Dec 2004 - 23:34:37 EST


On 19-Dec-04 Tim Churches wrote:
> Shawn Way wrote:
>

>>I've seen multiple comments about MS Excel's precision and accuracy.
>>Can you please point me in the right direction in locating information
>>about these?
>>  
>>

> As always, Google is your friend, but see for example
> http://www.nwpho.org.uk/sadb/Poisson%20CI%20in%20spreadsheets.pdf

There is a huge literature on this topic, some of it published in journals, much of it floating around on the web and in the archives of mailing lists.

Tim's reference above is interesting, but only one example. The McCullough and Wilson reference given there, though now somewhat dated, identifies many of the classic problems. Googling on

  mccullough wilson excel

will throw up a host of followups.

Informed statistical comment on the problems of Excel encountered by serious users can be found by browsing in the mailing list ASSUME (Association of Statistics Specialists Using Microsoft Excel):

  http://www.jiscmail.ac.uk/lists/assume.html

The most recent serious issue reported there is the RAND() bug: see ASSUME archives for Dec 2003 followed up in the March 2004 archives. The latter point to a statement from Microsoft:

  SYMPTOMS
  When you use the RAND function in Microsoft Office Excel 2003,   the RAND function may return negative numbers.   CAUSE
  This problem may occur when you try to use many random numbers,   and you update the RAND function multiple times. For example,   this problem may occur when you update your Excel worksheet by   pressing F9 ten times or more.
  RESOLUTION
  This problem is fixed in the Microsoft Excel 2003 Hotfix Package   that is dated February 29, 2004.

  http://support.microsoft.com/default.aspx?scid=kb;en-us;834520

  (and the deeper you probe in this, the worse it gets).

While using Excel for statistics has some limited value in the context of initiating to statistics students whose IT experience is limited to exposure to courses on Excel and Word, and the teacher wants to build on such experience, I think that Excel should never be used for serious statistical work, for several reasons.

  1. The many reported (and some allegedly fixed) bugs in calculation and algorithms necessarily provoke suspicion that others still exist or may have been introduced. One simply cannot trust the results without checking.
  2. Too many things can be done silently and invisibly, "behind the spreadsheet", by Excel. Changes to data and differences between what is shown on the spreadsheet and what goes into exported files can arise without the user being aware of them.

   A particularly frightening example is the "sort" disaster    reported to ASSUME (8 Dec 2003) by Allan Reese.

3. Excel has some value as a straightforward data entry pad.

   However, I have seen far too many cases where sloppy usage    has led to the resulting spreadsheet containing "information"    which is either superfluous or wrong, in ways which would    not be obvious to the user.

   For example, a "missing data" cell, if blank, may be interpreted    as having value zero. Some people enter "." for missing data,    but often are not consistent. If inadvertently a space is    entered in a cell outside the intended row/column range of the    data (or, I suspect, even if the spreadsheet cursor wanders    outside the range) then when the sheet is exported (e.g. as    "CSV") these extra rows and columns will be included.

   In one case I received an Excel spredsheet with hundreds of    such extra rows and dozens of extra columns, together with    dozens of cases where " " and "." had been used inconsistently,    all this over and over on each of about 6 "worksheet" pages;    not to mention data in the wrong columns etc. It took about 4    days of continuous work to clean this up.

   To be frank, for entering complex data the discipline enforced    by a properly designed Data Entry Form in a database package    would avoid such problems altogether, and such should be used.    The illusion of success that Excel gives the user is a most    treacherous danger and frankly I simply do not, in the first    instance, trust data in a spreadsheet.

4. The use of formulae in cells to generate cell values can

   cause all sorts of problems. One to especially watch out for    is that a formula may have been wrongly or inappropriately    "copied" from one column to another or from one worksheet    to another. You can of course check this by moving the cell    cursor to such cells and noting what the formula is, but as    you can imagine this is a horribly uhpleasant process (and    by the way take care that you don't inadvertently alter it    while you're doing this!). Also see Allan Reese's "sort"    disaster above, which was formula-induced.

I could go on. I've written at length already because many readers of R-help may be in situations where they necessarily receive data in Excel files, or have to use Excel themselves, and may not yet have become aware of the risks. So I'm writing as a warning to them: Don't trust Excel, but if you must use it then check everything, make sure it's what it should be, and make sure that it stays that way when the spreadsheet is accessed (as in (3) or (4) above, things may change invisibly).

Best wishes to all,
Ted.



E-Mail: (Ted Harding) <Ted.Harding@nessie.mcc.ac.uk> Fax-to-email: +44 (0)870 094 0861 [NB: New number!]
Date: 19-Dec-04                                       Time: 12:34:37
------------------------------ XFMail ------------------------------

______________________________________________
R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sun Dec 19 22:52:41 2004

This archive was generated by hypermail 2.1.8 : Fri 18 Mar 2005 - 01:23:46 EST