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Re: st: Replace with returned results in svy loop
From
Richard Williams <[email protected]>
To
[email protected], [email protected]
Subject
Re: st: Replace with returned results in svy loop
Date
Mon, 30 May 2011 13:59:03 -0500
At 09:59 AM 5/30/2011, Steven Samuels wrote:
Nick is right and I was wrong. The code does work with `r(N)'. I
apologize, but the question remains: what led to Kate's error message?
First, let me say that I hope Nick is having a good holiday weekend.
:-) Second, I don't think Kate showed us the final winning code. My
own guess was that the mysterious $fooditemall included a string
variable. Third, as a sidelight, while it may be fine in this
specific case, I try to avoid coding like priceperkilo !=. Better is
priceperkilo < . or else !missing(priceperkilo). This covers you in
case .a, .b, etc. have been used as missing data codes.
Steve
On May 30, 2011, at 1:45 AM, Nick Cox wrote:
I agree with Steve's general advice.
Actually, r(N) does have a local macro persona `r(N)' . If you use
`r(N)' the results will in this case be identical. Compare
. sysuse auto
(1978 Automobile Data)
. count if foreign
22
. di r(N)
22
. di `r(N)'
22
This is not illegal, but just a little indirect when a reference to
r(N) will do fine.
Note that the intermediate scalar is unnecessary and that, in any
case, the scalar created was -vmean- but there was a later attempt to
use -varmean-. So, the code could have been
foreach X of varlist $fooditemall {
quietly count if `X'==1 & priceperkilo != .
if r(N) > 0 {
display "Variable is: " "`X'"
svy, subpop(`X'): mean priceperkilo
mat meanpriceperkilo = e(b)
}
quietly count if `X'==1 & priceperkilo ==.
if `r(N)'>0 {
replace priceperkilo = meanpriceperkilo[1,1] if
`X'==1 & priceperkilo ==.
}
}
I can't verify that this is exactly as intended and I won't try to
undermine the strong advice that this is not a good idea.
I will mention a surprising initial assumption on this list that
"everyone" on this international list celebrates U.S. Memorial Day
weekend!
Nick
On Mon, May 30, 2011 at 4:56 AM, Steven Samuels <[email protected]> wrote:
> Kate Schneider:
>
> I would put it more strongly than Richard: substituting the mean
for all the data has no justification.
>
> If you are going to work in Stata, then I suggest you read the
error messages and do some digging. You haven't helped us by
showing (as requested in the FAQ) exactly what Stata typed, so we
could see where the error took place.
>
>
> But in your case, the error message makes the problem easy to
find: A type mismatch can occur only in some kind of comparison,
and you have only two lines that contain these: One is "if `X' ==
1 & priceperkilo !=.". But you already know that clause is
OK. That leaves: "if `r(N)'> 0". What could be wrong? If you
look at the -help- for "return", you will see that it never refers
to saved results as local macros. And only local macros get single
quotes around them (like `X'). If you didn't know that, but did a
Google search for "stata r(N)" you would be led to:
http://www.ats.ucla.edu/stat/stata/faq/returned_results.htm where
returned results are used without quotes. So the answer is: remove
the single quotes from around r(N).
>
> But although that solves the immediate problem, you are still
left with the fact that there is also no justification for
substituting the mean for missing values. There are two reasons.
First, mean substitution distorts most other sample statistics, for
example the standard deviation, skewness, kurtosis, some quantiles,
correlations with other variables. Second, there are good
imputation alternatives, as Richard has already stated.
>
>
> Steve
> [email protected]
>
>
> On May 29, 2011, at 10:16 PM, Richard Williams wrote:
>
> At 06:53 PM 5/29/2011, Kate Schneider wrote:
>> Thank you so much Richard! It totally works! We can't get it to only
>> replace the missing values, but I think it is more valid to create a
>> new variable with the estimated value anyways. Thanks so much!
>>
>> Kate
>
> Richard wrote:
>
> I'll take your word for it. :) I only looked at a small portion
of the code and didn't try to figure out the rest. I'd be careful,
though, if you aren't getting what you expected. You have to be
pretty lucky to accidentally get a result that is better than the
one you intended. Also, while I am not crazy about substituting the
mean for missing data, I am even less crazy about substituting the
mean for all the data, if that is in fact what you have done.
>
> Kate Schneider wrote:
> -Also, we have amended our code based on your recommendations but are
> getting a "type mismatch error"
>
> Here is what we're trying:
>
> foreach X of varlist $fooditemall {
> quietly count if `X'==1 & priceperkilo !=.
> if `r(N)'>0 {
> display "Variable is: " "`X'"
> svy, subpop(`X'): mean priceperkilo
> mat meanpriceperkilo = e(b)
> scalar vmean = meanpriceperkilo[1,1]
> }
> quietly count if `X'==1 & priceperkilo ==.
> if `r(N)'>0 {
> replace priceperkilo = varmean if `X'==1 & priceperkilo ==.
> }
> }
>
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-------------------------------------------
Richard Williams, Notre Dame Dept of Sociology
OFFICE: (574)631-6668, (574)631-6463
HOME: (574)289-5227
EMAIL: [email protected]
WWW: http://www.nd.edu/~rwilliam
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