st: RE: RE: Creating a group variable based on values in observations

["Sarah Edgington" <sedging@ucla.edu>]

Chris, 
I think there are a number of different ways to solve this problem.
How many markets are you dealing with?  If it's fewer than 20 here's a
solution that gets you around the reshaping issue.
First, create a new market id where market 1=1, market 2=10, market 3=100,
etc.  Then sum this id within days.  That will give you a group variable
where each place represents a particular market (starting with market 1 on
the right) and a 1 or 0 tells you if the market was open or not.  Your day
one group id would be 11111.  Day two's would be 10110.  

	gen double mid=10^(market-1)
	bysort day: egen double margroup=total(mid)

This only works well up to 19 markets because of precision issues.  In
principle, though, you could do it in any base and have everything add up to
create a unique group id.  So if you used 2 as your base instead of 10 (that
is, change the first line to  gen double mid=2^(market-1) ) you'd be able to
accommodate more markets.  Doing that you lose the ability to easily look at
it and read which markets are open straight from the group variable.  That
doesn't really matter for analytical purposes, though.

-Sarah


-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Chris Parker
Sent: Friday, May 20, 2011 3:05 PM
To: statalist@hsphsun2.harvard.edu
Subject: st: RE: Creating a group variable based on values in observations

Hi,

I think I have a solution. My data is a bit too big to do this all at once
(reshape gives a return code telling me productmarket takes on too many
values) but here is what works in case anyone runs into a similar
problem:

. gen marketdup = market
. reshape wide market, i(date) j(marketdup) . egen openmarkets =
concat(market*), punc(_) . encode openmarkets, gen(groupid) . drop
openmarkets . reshape long . drop marketdup

Chris

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Chris Parker
Sent: Friday, May 20, 2011 9:36 PM
To: statalist@hsphsun2.harvard.edu
Subject: st: Creating a group variable based on values in observations

Hi Statalist,

I have a problem that's been troubling me for a while now. I have daily
prices for several products in several markets over time. I use the data to
measure price dispersion as the coefficient of variation of prices on a day
for a product. However, not every market is open on every day.
Systematic differences between the markets that are open (such as average
distance between markets, percent of markets of type A, etc.) could impact
price dispersion, so I need to control for this. For each product I would
like to create a variable that lists which group of markets was open on each
day (openmarkets in the example below). I could then encode this variable
and include i.groupid which controls for these differences.

Example data for one of the products:

day	market		openmarkets	groupid
1	1		1-2-3-4-5	1
1	2		1-2-3-4-5	1
1	3		1-2-3-4-5	1
1	4		1-2-3-4-5	1
1	5		1-2-3-4-5	1
2	2		2-3-5		2
2	3		2-3-5		2
2	5		2-3-5		2
3	1		1-3-4-5		3
3	3		1-3-4-5		3
3	4		1-3-4-5		3
3	5		1-3-4-5		3
4	2		2-3-5		2
4	3		2-3-5		2
4	5		2-3-5		2

Any ideas?

Chris

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