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st: RE: How to know how a global macro was generated: -smr- as an example


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: How to know how a global macro was generated: -smr- as an example
Date   Thu, 11 Dec 2008 21:39:23 -0000

I guess I lay down the law on what is, and what is not, good practice so
frequently that I should be the last one to demur about somebody else
doing that. 

But I don't accept, personally, that all users who write Stata software
have an obligation to explain the formulae they use, even indirectly by
referencing. That is Utopian and impractical. (What if the references I
give do not explain all the ideas that I use, which will be very
common?)  

That aside, one good practice not followed here is to explain where the
(user-written) code discussed comes from. 

STB-29  sg29.1  . . . Tabulation of observed/expected ratios & conf.
intervals
        (help smrby if installed) . . . . . . . . . . . . . . . . . P.
Sasieni
        1/96    pp.21--24; STB Reprints Vol 5, pp.87--90
        improved version of smr

Looking at -smrby- I note that 

1. -smrby- itself does not define any globals. 

2. Just before they are used, -smrby- calls -_crccip-. 

3. _crccip.ado is part of the Stata distribution. Commands whose names
begin with _crc are short utilities written by the Stata developers.
(CRC is the abbreviation for the original name of StataCorp.) 

4. So look inside _crccip.ado. 

Nick 
n.j.cox@durham.ac.uk 

Tiago V. Pereira

Not rarely I almost drive myself crazy because several Stata commands
fail
to provide info about the formulations they use to compute
variances/standard errors in their calculations.

I have already claimed that most Stata packages should provide
references
on their formulae/and or to point out where we can get detailed info on
the methods implemented.

When no reference is given, I usually open the code and see how it was
calculated, trying to find ou in textbooks similar formulations.

However, in the case of the -smrby- command, which calculates the ratio
of
OBSERVED/EXPECTED counts and its confidence intervals, I could not
figure
out how the global macros S_1 and S_2 were computed.

Once you can rapidly take a look on the code of the -smr- command, I did
not paste the whole code here for simplicity.

Specifically, I am interested in the following lines:


if _rc==0 {
		_crccip `o' `level'
		local l=100*($S_1/`e')
		local u=100*($S_2/`e')
		local r=100*(`o'/`e')
		local emark ""
	}

I will be really grateful for any tip on how both $S_1 and $S_2 were
calculated.

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