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Re: st: Creating a second output data set


From   Bryan Sayer <bsayer@chrr.osu.edu>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Creating a second output data set
Date   Fri, 09 Sep 2011 21:07:31 -0400

No, if you look at the bottom of the message you will see an example.
The memory data set is composed of N observations. This is a probability proportional to size without replacement (stratified) sample. For the time being I am just doing one stratum at a time. See Levy and Lemeshow "Sampling of Populations" chapter 11.

Input (memory) data set (with marginal probability added)

District Size 	pi(i)
LUWEERO 12,466 	0.916858
KAMPALA 3,459 	0.542857
TORORO 	2,815 	0.448739
KAMULI 	549 	0.091546
Total 	19,289

Output (postfile) data set:

COMBINATIONS pi(I,j)
LUWEERO,KAMPALA 0.468854
LUWEERO,TORORO 0.377069
LUWEERO,KAMULI 0.070934
KAMPALA,TORORO 0.062531
KAMPALA,KAMULI 0.011473
TORORO,KAMULI 0.009139

Bryan Sayer



On 9/9/2011 8:13 PM, Steven Samuels wrote:


Bryan,

You don't need to create any loops. I assume that the  "memory" data
set  consists of  N x (N-1)/2 observations.    If you haven't done so
already, create two separate variables, e.g.  id_1  id__2 that
uniquely identify the  PSU pair. Call the joint probability  j_prob.
************************** sort id_1 id_2 **************************
will give data that look like:

id_1  id_2 j_prob 1      2    .07 1      3    .05 . . . N-1    N
.04

Then the following code will create the data set you are requesting.
******************************************* egen marg_prog =
total(j_prob), by(id_1) bys id_1: keep if _n==1 drop id_2 j_prob
save *******************************************

The same procedure will work if the output data set consists of any
number of tuples.  For 4-tuples, for example, change the -drop-
statement to:

******************** drop id_2 id_3 id_4 j_prob *****************

Out of curiosity: what selection algorithms are you studying?

Steve


On Sep 9, 2011, at 6:30 PM, Nick Cox wrote:

Too much survey sampling in this for me to understand what you want.
Someone else will probably help.


On Fri, Sep 9, 2011 at 11:08 PM, Bryan Sayer<bsayer@chrr.osu.edu>
wrote:
Great, thanks!  I tend to think more along the lines of FORTRAN.

A local for the total makes sense.  The total is used in the
calculation of the joint probability.

So locals seem to make sense for what I send to -postfile- (though
I realize I probably don't have it correct yet), provided it is ok
to change their value during the loops.  But maybe it should be
scalers?

But I am still stuck on the new variable I need to add to the input
data set (the one in memory).  I'm going to call that "memory data
set" to distinguish it from "postfile data set".

The memory data set is the input for this calculation.  Each
observation in the memory data set is one PSU.  The postfile data
set consists of each possible pair of PSUs from the memory data
set, thus the number of observations is the combination of _N taken
2 at a time from the memory data set (without replacement).  The
memory data set will gain one variable, the marginal probability
(margprob ), which is the sum of the joint probabilities involving
each PSU.

Can I act on margprob in the memory data set one observation at a
time?  I presume I would generate a new variable first, assigning
it a value of zero before the loops start

Basically, in the loop, margprob looks like this:

replace margprob[J] = margprob[J] + jointprob replace margprob[K] =
margprob[K] + jointprob

Where J and K are the observation number of the memory data set.

Does this work?

Bryan Sayer Monday to Friday, 8:30 to 5:00 Phone: (614) 442-7369
FAX:  (614) 442-7329 BSayer@chrr.osu.edu


On 9/9/2011 5:25 PM, Nick Cox wrote:

Your code shades between Stata and incomplete Stata, as you will
know.

However, a key principle here is that -postfile- never sees the
locals in your calling program. It just gets passed their values.
That's not a problem. It's the way to get round the basic fact
that one program's locals are invisible to another program.

Also these two lines definitely won't work

local N_total egen double `N_total'=total(`count')

The first defines the local N_total as blank, which is equivalent
to not defining it at all. So, Stata will read the second line
as

egen double = total(`count')

which will fail, as no new variable name is supplied.

That said, there is no need to create a variable just to hold a
total.

su `count', meanonly

will leave r(sum) in memory and the value of that can be put
somewhere appropriate, into a local or a scalar or directly into
another file.

On Fri, Sep 9, 2011 at 8:49 PM, Bryan Sayer<bsayer@chrr.osu.edu>
wrote:

So I am still a bit confused about how -postfile- works when I
want to preserve the data in memory.  Specifically, how I
generate the variables that I want in my -postfile- output
versus the new one I do want to add to the data set in memory.

I'm thinking I want to use a local (maybe macro?) variable for
my results that go to -postfile-?  In other words, how do I
distinguish variables between the two files.

Also, how do I accumulate results for my new variable that goes
in my memory data set.  I need to accumulate a sum for two
observations in memory on each post to -postfile-.

Here is what I have so far, but with the last part calculating
the marginal probability (note that the joint probability
calculation should be on one line):

program jointprob args design infile outfile psu count
margprob tempvar psu1 psu2 pi_one pi_joint tempfile results /*
set up the file with the joint probabilities */ postfile
`results' `psu1' `psu2' using "`outfile'" ,replace /* get the
number of observations and the total count */ local N=_N local
N_total egen double `N_total'=total(`count')

quietly { /* read the input data set and create combinations of
N items taken 2 at a time, without replacement */ forvalues J =
1/`N'{ forvalues K = 1/`N'{ if `K'>`J'{ psu1=`psu'[`J']
psu2=`psu'[`K']

pi_joint=(`count[`J']'*`count[`K']'/`N_total') *
((1/(`N_total'-`count[`J']')+(1/(`N_total'-`count[`K']')) post
`results' psu1 psu2 pi_joint } } } }


Bryan Sayer Monday to Friday, 8:30 to 5:00 Phone: (614)
442-7369 FAX:  (614) 442-7329 BSayer@chrr.osu.edu


On 9/7/2011 9:44 AM, Roger Newson wrote:

-postfile- will still work if there is an existing dataset in
the memory. However, the new dataset will be built in a
file.

Best wishes

Roger


Roger B Newson BSc MSc DPhil Lecturer in Medical Statistics
Respiratory Epidemiology and Public Health Group National
Heart and Lung Institute Imperial College London Royal
Brompton Campus Room 33, Emmanuel Kaye Building 1B Manresa
Road London SW3 6LR UNITED KINGDOM Tel: +44 (0)20 7352 8121
ext 3381 Fax: +44 (0)20 7351 8322 Email:
r.newson@imperial.ac.uk Web page:
http://www.imperial.ac.uk/nhli/r.newson/ Departmental Web
page:


http://www1.imperial.ac.uk/medicine/about/divisions/nhli/respiration/popgenetics/reph/




Opinions expressed are those of the author, not of the institution.

On 07/09/2011 14:40, Bryan Sayer wrote:

-postfile- will post my results, but my reading of how it
works seems to indicate that my original data set cannot be
open at the same time. The examples appear to me to clear
the existing data set from memory.

Admittedly, this is without me having tried anything yet,
but am I not reading it correctly?

What I need to do is a double loop through the input data
set, outputting a record on each iteration of each loop. So
I need the input data set open in memory, and a second file
to post the results to.

Are there any examples of something similar?

Thanks!

Bryan Sayer Monday to Friday, 8:30 to 5:00 Phone: (614)
442-7369 FAX: (614) 442-7329 BSayer@chrr.osu.edu


On 9/6/2011 4:58 PM, Roger Newson wrote:

I think you are looking for the -postfile- utility. In
Stata, type

help postfile

to find out more.

HTH.

Best wishes

Roger


Roger B Newson BSc MSc DPhil Lecturer in Medical
Statistics Respiratory Epidemiology and Public Health
Group National Heart and Lung Institute Imperial College
London Royal Brompton Campus Room 33, Emmanuel Kaye
Building 1B Manresa Road London SW3 6LR UNITED KINGDOM
Tel: +44 (0)20 7352 8121 ext 3381 Fax: +44 (0)20 7351
8322 Email: r.newson@imperial.ac.uk Web page:
http://www.imperial.ac.uk/nhli/r.newson/ Departmental Web
page:


http://www1.imperial.ac.uk/medicine/about/divisions/nhli/respiration/popgenetics/reph/






Opinions expressed are those of the author, not of the institution.

On 06/09/2011 21:53, Bryan Sayer wrote:

I need to create an output data set that will differ in
the content and number of observations from the input
file. The observations will be created one at a time,
based on the input data set.

Specifically, I am creating all combinations of N
objects taken two at a time. I will probably also do
permutations.

The input data set (to start with) consists of N
records with two variables, the primary sampling unit
(PSU) and a size variable associated with the PSU (a
count variable). I want to create two output data sets.
One is each combination of PSU with the associated
joint probability. The second has the same structure as
the input data set but includes the marginal
probability, calculated as the sum of the joint
probabilities associated with the PSU (which are
accumulated as each combination is created).

The part I am stuck on is how to output the data set of
combinations. Can someone point me to a program that
outputs a file as calculations are made?

(For those interested, this is for probability
proportional to size (PPS) sampling. See, for example,
Levy and Lemeshow "Sampling of Populations, chapter
11).

Here is an example of one stratum:

Input data set (with marginal probability added)

District Size pi(i) LUWEERO 12,466 0.916858 KAMPALA
3,459 0.542857 TORORO 2,815 0.448739 KAMULI 549
0.091546 Total 19,289


Output data set:

COMBINATIONS pi(I,j) LUWEERO,KAMPALA 0.468854
LUWEERO,TORORO 0.377069 LUWEERO,KAMULI 0.070934
KAMPALA,TORORO 0.062531 KAMPALA,KAMULI 0.011473
TORORO,KAMULI 0.009139



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