Matt (& Ariel)

One more follow-up question...

I have been matching exactly on sale year, wanting to ensure that the treated pv home sales occur in the same year as the matching comparables. Ideally, I would not match exactly on sale year, but rather the difference between the treated sale date and the comparable sale dates. So, say, for example, I would restrict my comparables to only those with a sale date within 6 months, either before or after (i.e., +/- 0.5 years). This is more like a nearest neighbor matching, and would avoid missing the matches that are within a few months of each other, but in different years.

Because this is such a different approach than coarsening the data, I had not considered it, but now wonder if there was a work around you have considered and could propose for this situation.

Thanks, in advance,

Ben

Ben Hoen

LBNL

Office: 845-758-1896

Cell: 718-812-7589

From: Matt Blackwell [mailto:m.blackwell@rochester.edu]
Sent: Tuesday, July 08, 2014 1:36 PM
To: Ariel Linden
Cc: Ben Hoen; cem@lists.gking.harvard.edu
Subject: Re: [cem] Understaning CEM's use of a categorical variable and #0

Hi Ben,

Ah, taking a look, I've figured it out. First, the one you want to use is the "bgcode." This is for two reasons. First, as you've guessed, CEM doesn't work with string variables, only numerics (I had forgotten this in my last reply). Second, the "bgnum" variable also trips us Stata because of the size of the numbers. In the CEM internals, Stata is treating two numbers with the same scientific notation (1.2e+12) as the same. This is why there are more matches with that version than the other. Thus, your best bet is the "bgcode." Hope that helps and sorry for any confusion.

Cheers,

Matt

~~~~~~~~~~~

Matthew Blackwell

Assistant Professor of Government

Harvard University

url: http://www.mattblackwell.org

On Tue, Jul 8, 2014 at 11:16 AM, Ariel Linden <ariel.linden@gmail.com> wrote:

Ben – quick response to your last question about missing values: if CEM excludes units with missing values (Matt can clarify), you can generate a missing value indicator for the given variable and use that in the matching procedure. So you’d basically be matching on the pattern of missingness of that variable. A more comprehensive approach would be to use –mi- to impute missing values (CEM can be run on datasets that were generated for multiple imputation – see Matt’s paper in the Stata Journal for discussion).

From: cem-bounces@lists.gking.harvard.edu [mailto:cem-bounces@lists.gking.harvard.edu] On Behalf Of Ben Hoen
Sent: Tuesday, July 08, 2014 10:30 AM
To: 'Matt Blackwell'

Cc: cem@lists.gking.harvard.edu
Subject: Re: [cem] Understaning CEM's use of a categorical variable and #0

Hi Matt (& Ariel)

I have dug a bit more into this and am even more confused. I am leaving, for the time being, the issue of larger or smaller geographies, and instead am focusing on variable form (e.g., text vs. numerical).

I ran an experiment to test a few things, and have attached the output from that (as well as the sample dataset FYI, if you wanted to try to duplicate the experiment).

The research question was the following: does cem care if a variable is entered as a string, an ordinal long integer or a non-ordinal long integer (assuming each variable has the same number of unique values)?

To test this I ran cem three times once each for blockgroup as a text variable (bgname, used in cem1), as an encoded ordinal variable (bgcode, used in cem2), and as a non-ordinal variable (bgnum, used in cem3). In addition to the block group I include the variables we discussed previously, namely sfla, age, acres and saleyear. In each case I am matching exactly on blockgroup (and saleyear), by using the “(#0)” syntax.

Prior to this I used codebook to examine the three variables (and the other variables in cem) and see that in each case the blockgroup variables have 359 unique values and no missing values for the variable.

When running cem I find that cem1 has 333 strata, cem2 has 5283 strata, and cem3 has 1510 strata. Of course, when there are more strata, there are fewer matches, so cem1 produces 425 matching treated cases, cem2 277 and cem3 384.

Why is it that cem is treating these three forms of the same variable so differently?

Separately, I have an additional variable, which I left out, but which has some missing values. If I show the breaks for this variables as: “(0 10 25 50 90)” does cem create an additional strata for missing values? If not, is there a way to do this, while still maintaining (some) control of the breaks?

Thanks, for all you help with this. cem is a great program and has aided me in my work tremendously.

Ben

Ben Hoen

LBNL

Office: 845-758-1896

Cell: 718-812-7589

From: Matt Blackwell [mailto:m.blackwell@rochester.edu]
Sent: Monday, July 07, 2014 10:36 PM
To: Ben Hoen
Cc: cem@lists.gking.harvard.edu
Subject: Re: [cem] Understaning CEM's use of a categorical variable and #0

Hi Ben,

My immediate guess would be the missing data on the county variable, which may be interacting strangely with the string variables. Maybe try two things: 1) creating numeric versions of both and repeat the matches and 2) try dropping the missing county observations and comparing the matches then.

Cheers,

Matt

On Mon, Jul 7, 2014 at 10:17 PM, Ben Hoen <bhoen@lbl.gov> wrote:

Just realized that blockgroup and county are both strings. See below:

That likely is NOT what cem is looking for is it? Source of the problem?

(And yes, block group variable, which is the census number, is unique across counties)

Ben

Ben Hoen

LBNL

Office: 845-758-1896

Cell: 718-812-7589

From: Matt Blackwell [mailto:m.blackwell@rochester.edu]
Sent: Monday, July 07, 2014 10:10 PM
To: Ben Hoen
Cc: cem@lists.gking.harvard.edu
Subject: Re: [cem] Understaning CEM's use of a categorical variable and #0

Hi Ben,

Hm, it definitely should produce more matches when you use county. One possible issue that I can think of off the top of my head is this: is the block group variable unique across counties/states? Or do the values of the block group variable repeat? One thing to check is to see if what happens if you exact match on both the county and the block group in a single match.

Hope that helps! If it doesn't, definitely let us know.

Cheers,

Matt

~~~~~~~~~~~

Matthew Blackwell

Assistant Professor of Government

Harvard University

url: http://www.mattblackwell.org

On Mon, Jul 7, 2014 at 9:36 PM, Ben Hoen <bhoen@lbl.gov> wrote:

Hi all,

I have been using the program cem in Stata (Version 13 MP, with Windows 7 Pro 64 bit), and thought I understood what it was doing well enough but today something occurred which surprised (read worried) me, in that it acted as I would NOT have expected it to.

I am trying to match target (i.e,, treated) homes to similar (i.e., "comparable") homes that do not have the treatment. In this case, the "treatment" is whether the home does or does not have a photovoltaic energy system (pv). I have 100 pv homes (treated), and ~ 5,000 non-pv homes (comparable).

To match these homes I am using some basic characteristics of the home - e.g., square feet of living space (sfla), size of the parcel (acres), age of the home (age), as well as the year in which it sold (sale year) to ensure the comparable home sold in the same year as the target home and, finally, a geographic variable (such as the block group) to ensure the comparable home is located in the same geography. For sale year and the geogrpahy, they must match perfectly; i.e., the comparable homes must have sold in the same year as the target (pv) home and also be located in the same geography. For the purposes of this discussion those geographies could be either the census block group (blockgroup) or the county (county). All of the block groups fall within the counties, and there are many more block groups than counties delineated in the data. For example, I have approximately 30 block groups (each with at least one treated and one comparable case) and 10 counties (each with at least one treated and one comparable). In practice, though, in most geographies I have ~ 20-50 times the number of pv homes available as comparables to match to.

Using the sample data and talking to local experts, I have established appropriate cut points for my various characteristics and run a command similar to the following, when blockgroup is used as the geography:

cem sfla(0 1000 2000 3000 5000) age(0 1 10 20 100) acres(0.05 0.15 0.5 1 10) saleyear(#0) blockgroup(#0) , treatment(pv)

And the following, when county is used as the geography:

cem sfla(0 1000 2000 3000 5000) age(0 1 10 20 100) acres(0.05 0.15 0.5 1 10) saleyear(#0) county(#0) , treatment(pv)

So, here's the confusing part:

I will have ~ 70 matching pv homes, and 300 comparable homes if blockgroup is used, but only 20 matching pv homes, and 100 comparables homes if county is used. In other words, when I allow a broader geography of comparables to be drawn from, I get fewer matching cases. i would think the exact opposite would be the case; if a cast a broader geographic net, I would have more matches not less.

Any ideas why this would occur?

Thanks, in advance, for any insight you could offer.

Ben
Berkeley Lab

Ben Hoen

Staff Research Associate

Lawrence Berkeley National Laboratory

Office: 845-758-1896

Cell: 718-812-7589

bhoen@lbl.gov

http://emp.lbl.gov/staff/ben-hoen

Visit our publications at:

http://emp.lbl.gov/reports/re

https://spreadsheets.google.com/a/lbl.gov/spreadsheet/viewform?formkey=dGlFS1U1NFlUNzQ1TlBHSzY2VGZuN1E6MQ

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