RE: st: imputing continuous values when respondents select categories, e.g., income category

[Richard Williams <Richard.A.Williams.5@ND.edu>]

At 11:20 PM 4/24/2009, Roy Wada wrote:

> ystar(a,b) will still give you censored predictions, which
> may not be a good idea as Richard indicated.
>
> Anyone knows if it's okay to use non-censored predictions from
> -intreg- as a part of 2SLS and bootstrap the standard error,
> assuming we have identifying instruments in the first stage?
>
> Roy

These possibilities are starting to make my head hurt.  :)  To back 
up, I've never heard of anyone computing the predicted values from 
intreg and then using them as an independent variable in subsequent 
analyses.  That may just reflect my ignorance, but it seems like at a 
minimum your standard errors would be too optimistic.

For that matter, there are concerns about using intreg for dependent 
variables - if the assumptions of the method are not met (e.g. 
normality) the estimates may be wrong.  And, as the manual points 
out, for something like income, you may want to use the logged values 
of the interval endpoints.  See the manual for an example.  So, you 
have to be careful that your use of intreg is legit in the first place.

Remember, too, that unlike missing data fill in the blank techniques, 
with intreg you aren't just imputing some values, you are imputing 
all of them.  And, if you are computing this y-hat from x1, x2 and 
x3, why not just use x1, x2 and x3 in your other models and leave out 
the y-hat?  Remember, the y-hat will be perfectly correlated with x1, 
x2 and x3 because it is computed from them.

I'm just improvising here, but this doesn't seem like the way intreg 
should be used.  If its assumptions are met, it can be a very nice 
alternative to other ordinal methods.  But, trying to use the 
estimated values from it as independent variables seems 
problematic.  It is like the problems you have with single imputation 
of missing data, but even worse since every value is being imputed.

I keep wishing Scott Long or somebody like that would write more 
about intreg, so if we have any experts out there on it feel free to chime in!


-------------------------------------------
Richard Williams, Notre Dame Dept of Sociology
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EMAIL:  Richard.A.Williams.5@ND.Edu
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