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# Re: st: Question about average marginal effects for logit models with dummy variables

 From Richard Williams To statalist@hsphsun2.harvard.edu, statalist@hsphsun2.harvard.edu Subject Re: st: Question about average marginal effects for logit models with dummy variables Date Mon, 08 Oct 2012 07:14:37 -0500

```At 05:45 AM 10/8/2012, Tirthankar Chakravarty wrote:
```
```-margins- inherits whether variables are discrete or continuous from
the estimation command that precedes it (in your case -logit-). In
turn, the estimation command does its best to gauge whether included
variables are discrete or continuous.
```
```
```
The part about the estimation command isn't quite right. Even if a variable is coded 0/1 margins will treat it as continuous unless you use factor variable notation. As Nick Cox explained once, margins needs to know if a variable is discrete, as opposed to, say, being a continuous variable that only happens to take on the values of 0 and 1 in the sample.
```
```
```To change the default behaviour, or indeed to be cautious, it is
advisable to use the unary operators to denote continuous and
indicator variables, "c." and "i." (as documented in -help fvvarlist-)
liberally.
```
```
```
Again, in the absence of any factor variable notation, the default behavior of margins is to assume variables are continuous. mfx, on the other hand, would assume a variable coded 0/1 was discrete. It is more than just being cautious -- margins won't handle discrete variables correctly if you are not using factor variable notation. If x1 is coded 0/1, the estimation command won't care if you refer to it as x1 or i.x1, but margins will.
```

```
```The documentation for the -margins- command goes into great details on
these matters (-help margins-).

T

On Mon, Oct 8, 2012 at 3:41 PM, Bob Reed <bob.reed@canterbury.ac.nz> wrote:
```
> Hi, When Stata calculates AMEs for logit models with dummy variables, does it "know" that the dummy variable is discrete, and calculate its marginal effect accordingly?
```>
> Bob Reed
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