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st: creating cross tables/ matrices with expected/ observed frequencies from long data set


From   Nick Darson <nick.darson@googlemail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: creating cross tables/ matrices with expected/ observed frequencies from long data set
Date   Sat, 22 Sep 2012 10:35:49 +1000

Dear Statalisters,
I would like to create several tables/matrices with expected
frequencies and observations (to be able to carry out a Chi Square
test of how well my model describes the data).
I have the following data set (each person chose from 2 sets, the
first one containing 3 option, the second one containing 4 options),
with ID=individuals (300), set=choice set, option= option, choice=
chosen option (dummy), Prob= expected probability for each individual
based on my random-effects logit model.

ID	Set	Option    Choice		Prob
1	1	A		0		0.2
1	1	B		1		0.7
1	1	C		0		0.1
1	2	D		0		0.1
1	2	E		0		0.2
1	2	F		1		0.4
1	2	G		0		0.3
2….

First I would need a cross-table with observed frequencies for the two
choice sets (looking as follows):
      A  B  C
D
E
F
G

I am familiar with the tab-command, but a bit lost how do to obtain
the table given my data-set arrangement? Any help would be
appreciated.


Then I would need the same table as above, but with expected (joint)
probabilities instead of observed frequency. For this, I had the
following strategy in mind:
1.) Create 2 matrices, one for each choice set
2.) Transpose one matrix and multiply to obtain joint probability

For 1.), I would need one table with the probabilities listed (column=
Individuals; rows= options) as follows:

        ID1    ID2 	 ID3 …..
A     0.2	   …..					
B     0.7						
C    0.1						
(and same table/ matrix for set 2).
How would I do this in Stata? To my understanding, Tabs and tabstats
only provide summaries etc, but I want the original value of prob
listed?
Any help would be appreciated!
BTW: this is a simplification of the original data set (more sets and
options) and I would like to do several things in one set…therefore, I
“keep”/”reshape” combos would not be suitable.

Cheers
Nick

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