> -----Original Message-----
> From: Richard Herrell [mailto:[email protected]]
> Sent: Wednesday, November 27, 2002 3:56 PM
> To: [email protected]
> Subject: RE: st: STATA Vs. SAS
>
>
>
> On Wed, 27 Nov 2002, Nick Cox wrote:
>
> > I am still not clear whether -x- and -y- are variables,
>
> Yup, x and y are variables with values for each observation
> in the data
> set.
>
> > and whether this all -- in Some Alternative
> > Software -- is tacitly looping over
> > observations. (In my case, please assume
> > complete ignorance of SAS.)
>
> In SAS (and dBase or SPSS, e.g.), these commands are
> evaluated for each
> observation. In such a structure, once a true condition is met,
> subsequent conditions are skipped. Say I want to assign grades to my
> students' percentage scores. In SAS I could write
>
> if score > 90 then grade='A';
> else if score > 80 then grade='B';
> else if score > 70 then grade='C';
> else if score > 60 then grade='D';
>
> SAS will assign the appropriate grade to the first condition
> that is true,
> then skip the following conditions in the structure. (This
> is also true
> of other database management languages I've used such as
> dBase.) This is
> why I find it more efficient and less cumbersome than writing
> the complete
> condition for a series of "if ... then"s when every one will
> be evaluated.
Much of this probably boils down to what you learned first, and
therefore what seems "natural". Leaving efficiency aside for a moment,
though, it does seem worth pointing out that in Stata you need not have
syntax any more complex than the syntax listed above:
generate grade="D" if score > 60
replace grade="C" if score > 70
replace grade="B" if score > 80
replace grade="A" if score > 90
That is, in this case, by reversing the order, you can avoid saying
things like
replace grade="B" if score>80 & score<=90
and so on.
Hugely complex nested conditions might be easier in SAS. It could be
argued that
if cond1
if condA
then x=1
else if condB
then x=2
else if condC
then x=3
else if cond2
if condA
then x=4
if condB
then x=5
...
displays the structure more clearly than
gen x=1 if cond1 & condA
replace x=2 if cond1 & condB
replace x=3 if cond1 & condC
replace x=4 if cond2 & condD
......
The downside is that if you don't indent carefully, and/or if you miss
an else, or whatever, then it might not be transparent exactly what set
of conditions hold at a given point in the SAS massive if/then/else
structure.
--Nick Winter
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