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From |
"Nick Winter" <nwinter@policystudies.com> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
RE: st: STATA Vs. SAS |

Date |
Wed, 27 Nov 2002 16:04:48 -0500 |

> -----Original Message----- > From: Richard Herrell [mailto:rherre2@uic.edu] > Sent: Wednesday, November 27, 2002 3:56 PM > To: statalist@hsphsun2.harvard.edu > 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 * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**RE: st: STATA Vs. SAS***From:*Richard Herrell <rherre2@uic.edu>

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