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st: R: RE: Simple tab needed but multiple records+How do people learn Stata


From   "Carlo Lazzaro" <carlo.lazzaro@tin.it>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: R: RE: Simple tab needed but multiple records+How do people learn Stata
Date   Sun, 7 Oct 2007 23:49:08 +0200

Dear Nick,

you are as usual right.

I tried to reply to Joseph' thread making (probably too) basic assumption.

However, at the beginning of the current year when I started using Stata (as
Statalisters may know I am an Italian health economist, not that experienced
in statistics, by the way) I was not able to even figure out such a trivial
posting.

Even though it may sound like an advertising stunt, using Stata and the
invaluable chance to address some questions to more experienced Statalisters
changed to a remarkable extent the way I am now approaching researching and
consulting in health economics.
Besides, I do enjoy using Stata as well as this "thinking outside the box"
-with respect to my recent "quantitative past"- that Stata supports.

May my humble experience contribute to the surely endless story on "How do
people learn Stata?".

Kind Regards,

Carlo a


-----Messaggio originale-----
Da: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] Per conto di Nick Cox
Inviato: domenica 7 ottobre 2007 19.51
A: statalist@hsphsun2.harvard.edu
Oggetto: st: RE: Simple tab needed but multiple records

Maarten is right. If data are like this 


id   female likes_cats likes_dogs 
--   ------  ---------- ----------
1      0         0          0
2      1         1          0 
3      0         1          0 
...

in which each person is represented by only one
observation (record), then it's easy to count 
how many people satisfy two (or indeed more)
different conditions. 

e.g. -count if female & likes_cats & likes_dogs-

Nor are indicators (dummy, logical, Boolean 
variables) essential as we can always use explicit 
true or false conditions instead. 

This kind of structure is I think also 
assumed by Carlo Lazzaro in his posting in 
this thread. 

However, 

This is not the structure Joseph 
has and it would be unnatural to force
his dataset into a different structure
given the irregularity of dates that 
he presumably has. 

Hence Kit's proposal is closer to, indeed
on, the mark. 

What's more, this is essentially the same 
problem as that posted by Paul O'Brien
just the same day and already replied to
with code 

<http://www.hsph.harvard.edu/cgi-bin/lwgate/STATALIST/archives/statalist.071
0/date/article-161.html>

The class of problem is this: 

1. There is some kind of grouping, most obviously into panel 
or longitudinal data. For concreteness, we'll talk "panels" and 
remember that the idea is more general. (Indeed, no 
kind of time basis, regular or irregular, is essential here.) 

2. Hence, multiple observations for each panel
are likely. 

3. Some question arises about panels that requires 
comparison of different observations. 

4. For each observation, we can say whether 
it satisfies some condition. That is a true-or-false
calculation. 

5. We need to summarise that true-or-false result
over all observations in each panel. This can be done with 
-egen, by(<panelid>)- or -by <panelid>: egen- or 
-by <panelid>: gen-. 

6. Then we need to combine information on different
conditions using logical operators such as &, | and !. 

7. Finally, we must count panels, not individuals. 

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

Kit Baum 
---------------------------------------------
I think this should work, without the necessity of reshaping:

bysort id: gen early = inrange(age, 17, 25)
by id: gen late = age > 30
by id: gen both = cond(_n==_N, (sum(early) & sum(late)) , .)
count if both == 1

To test,

set obs 1000
g id=mod(_n,100)+1
g age=40*uniform()

Maarten Buis
--------------------------------------------
This kind of problem usually becomes a lot easier when you first use
-reshape- to put the data into wide format.

Joseph Wagner
--------------------------------------------
> I have a dataset of x-ray records with multiple records per 
> patient.  The records consist of id, age, and sex and I need 
> to know how many persons had an x-ray when they were between 
> the age of 17 and 25 AND when when they were over 30.  

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