Date: Wed, 3 May 2000 18:41:19 -0700
Reply-To: latuttle <latuttle@EMAIL.MSN.COM>
Sender: "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From: latuttle <latuttle@EMAIL.MSN.COM>
Subject: Re: Problem wit fct mean in sas 7 on Unix?
Maybe I am missing something.
Assume for a moment that my_xtot and xtot are exactly equal.
338 my_cond = my_xtot > 6;
339 cond = xtot > 6;
Won't these statements resolve as:
my_cond= (not true, returns 0);
cond= (not true, returns 0);
thus my_cond and cond will always be zero when the two 'averages' are
perfectly equal?
If you run this data step...
349 data test;
350 set mydata;
351 if my_xtot ne xtot;
352 run;
NOTE: There were 38 observations read from the dataset WORK.MYDATA.
NOTE: The data set WORK.TEST has 0 observations and 27 variables.
NOTE: DATA statement used:
real time 0.04 seconds
... you get an empty dataset because the two 'means' are exactly equal.
Laura
<belislep@my-deja.com> wrote in message news:8epu1a$lu3$1@nnrp1.deja.com...
> Dear Sas users,
>
> I recently experienced a 'funny' problem due to use of function
> mean() with sas 7, which I use on a Unix system.
>
> It might not be a new fact to most of you, but I would like to know
> what you think of the potential precision problems of mean().
>
> At the bottom of this message, I will give a short program along with
> data, in which each subject should have a score (that score being the
> average of 21 subscales) of 6. However, it turns out that all
> subjects, when using the function mean(), do not have a score
> of EXACTLY 6, as you can see from the logical variables computed
> (cond*).
>
> This problem might be regarded as negligeable as it will not make any
> practical difference in most day-to-day applications, but suppose
> you want to do some sub-analyses for subjects with score > 6;
> in the example given below, none fo the patients in this list should
> be part of that sub-analysis, but it will not be the case...
>
> Was this bug ever reported? Should I write to SAS technical support?
> Thanks for your thoughts.
>
> Patrick Belisle
> Div. of Clinical Epidemiology
> Montreal General Hospital
> Montreal, Qc, CAN
>
> --
>
> data mydata;
> input x1-x21;
>
> cards;
> 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 3
> 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 1 3
> 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 1 3
> 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 1 1 3
> 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0 1 3
> 0 1 0 0 0 1 0 1 0 0 0 0 1 0 0 0 0 1 0 0 1
> 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 1 0 2 0 1 0
> 0 0 0 1 0 0 0 0 0 0 0 0 0 0 1 1 0 1 0 1 1
> 0 0 0 0 0 0 0 0 0 0 1 0 1 0 1 1 1 0 1 0 0
> 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 3 0 1 0
> 0 0 1 0 0 0 0 0 0 0 0 0 0 0 1 2 0 0 0 1 1
> 0 1 0 1 0 0 0 0 0 0 1 0 0 0 1 1 0 0 0 1 0
> 0 0 0 0 0 0 0 1 0 0 1 0 1 0 1 0 0 0 0 1 1
> 0 0 0 0 0 0 0 0 0 0 1 0 0 0 1 1 1 1 0 1 0
> 0 0 0 1 0 0 0 0 0 0 1 0 0 0 1 0 1 0 1 1 0
> 0 0 0 0 0 0 0 0 0 1 1 0 0 0 1 1 1 0 0 1 0
> 0 0 0 0 0 0 0 0 0 0 1 0 1 0 1 0 1 0 1 1 0
> 0 0 0 1 0 0 0 1 0 0 0 0 0 0 1 1 0 0 1 1 0
> 0 0 0 1 1 0 0 1 0 0 0 0 0 1 1 0 0 0 1 0 0
> 1 0 0 0 0 0 0 0 0 0 1 0 0 0 1 0 0 1 0 1 1
> 1 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 1 1 0 1 0
> 0 0 0 0 0 0 0 1 0 0 1 0 0 0 1 1 0 1 0 1 0
> 0 0 0 1 0 0 0 0 0 0 1 0 0 0 1 1 0 1 0 1 0
> 0 0 0 1 0 0 0 0 0 0 0 1 1 0 0 0 0 0 1 1 1
> 1 0 0 1 0 0 0 0 0 1 0 0 0 1 0 0 0 1 0 1 0
> 2 0 0 0 0 1 0 0 0 1 0 0 0 0 0 1 0 1 0 0 0
> 0 0 0 1 0 0 1 1 0 0 1 0 0 0 1 0 0 0 0 1 0
> 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 1 1 1
> 0 0 0 0 0 0 0 1 0 0 1 0 0 0 1 0 0 1 0 1 1
> 0 1 0 1 0 0 0 0 0 0 0 0 2 0 0 1 0 0 0 1 0
> 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 1 0 0 2 0
> 0 0 0 0 1 0 0 0 0 0 0 0 0 0 1 1 0 0 1 1 1
> 0 1 0 1 0 0 0 0 0 0 0 1 0 1 1 0 0 0 0 1 0
> 0 0 0 0 0 0 0 1 0 0 0 0 1 1 0 1 1 0 0 0 1
> 0 0 0 0 1 0 0 1 0 0 0 0 0 0 1 3 0 0 0 0 0
> 0 0 0 1 0 0 0 1 0 0 1 1 0 0 1 0 1 0 0 0 0
> 0 0 0 0 0 0 0 0 0 0 1 0 0 0 1 1 0 0 0 1 2
> 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 2 0 0 0 0 2
> ;
>
> data mydata;
> set mydata;
>
> x_sum = sum(of x1-x21);
> x_n = N(of x1-x21);
> my_xtot = 21 * (x_sum / x_n);
> xtot = 21 * mean(of x1-x21);
>
> format my_xtot xtot
> 22.20;
>
>
> my_cond = my_xtot > 6;
> cond = xtot > 6;
>
>
> proc freq;
> tables my_xtot xtot;
> tables my_xtot * my_cond / list;
> tables xtot * cond / list;
> run;
>
> --
>
> Outputs excerpt:
>
> Cumulative Cumulative
> MY_XTOT MY_COND Frequency Percent Frequency Percent
> ----------------------------------------------------------------------
> 6.00000000000000 0 38 100.00 38 100.00
>
> As you can see, the part above is correct (computing the mean 'myself'),
> but the part below isn't (using mean() to compute the score)...
>
> Cumulative Cumulative
> XTOT COND Frequency Percent Frequency Percent
>
> ------------------------------------------------------------------------
> 5.99999999999999 0 5 13.16 5 13.16
> 6.00000000000000 0 31 81.58 36 94.74
> 6.00000000000000 1 2 5.26 38 100.00
>
>
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> Before you buy.
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