<>
All of which makes me think that there is a strong case - even taking into
account Stata`s reluctance to bombard users with warnings - to alert the
user to the drop and why it occurred. Why does this code (see [R], p. 907)
***
sysuse auto, clear
logit foreign mpg weight gear_ratio if !(foreign==0 & gear_ratio > 3.1)
***
trigger a warning " Note: 4 failures and 0 successes completely determined."
and -rreg- does not?
HTH
Martin
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Martin Weiss
Sent: Samstag, 7. November 2009 15:19
To: [email protected]
Subject: st: RE: RE: RE: Possible bug
<>
So here is an illustration of the -drop- behavior of -rreg-: I increase the
outlier quality of observation # 23 in the auto dataset, first by
multiplying it by 1.5. In this case, as the -summarize- command shows, its
cooks D stays below 1, so -rreg- uses all observations.
In the last example, I increase the multiplier to 1.6, cooks D exceeds 1,
and number 23 now goes unused in -rreg-, as the last -list- command shows.
You may want to check this for your dataset...
*******
sysuse auto, clear
//normal case: no drop in -rreg-
qui{
reg mpg weight foreign
predict cooksdist, cooksd
su cooksdist, det
}
qreg mpg weight foreign, nolog
rreg mpg weight foreign, nolog
//multiply one obs by 1.5
//no drop in -rreg- yet as max(cooks D)
//still below 1
replace weight=1.5*weight in 23
qui{
reg mpg weight foreign
capt drop cooksdist
predict cooksdist, cooksd
noi su cooksdist, det
}
qreg mpg weight foreign, nolog
rreg mpg weight foreign, nolog
//reload autos to start anew
sysuse auto, clear
//multiply one obs by 1.6
//now drop in -rreg- yet as max(cooks D)
//exceeds 1
replace weight=1.6*weight in 23
qui{
reg mpg weight foreign
capt drop cooksdist
predict cooksdist, cooksd
noi su cooksdist, det
}
qreg mpg weight foreign, nolog
rreg mpg weight foreign, nolog
//see unused obs (23)
l if !e(sample)
*******
HTH
Martin
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Apostolos Ballas
Sent: Samstag, 7. November 2009 13:57
To: [email protected]
Subject: st: RE: RE: Possible bug
Thanks for the suggestion. You are quite right - I cannot reproduce it. The
observation that is being dropped has the max value in one of the
independent variables. Is there an explanation for this?
Apostolos
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Martin Weiss
Sent: Saturday, November 07, 2009 2:36 PM
To: [email protected]
Subject: st: RE: Possible bug
<>
Can you reproduce this with a built-in dataset? I cannot:
*******
sysuse auto, clear
rreg mpg weight foreign, nolog
qreg mpg weight foreign, nolog
rreg mpg weight foreign length turn, nolog qreg mpg weight foreign length
turn, nolog
*******
Also capture the one observation that -rreg- omits via -l if !e(sample)-
after estimation of -rreg-, and see what is special about it...
HTH
Martin
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Apostolos Ballas
Sent: Samstag, 7. November 2009 08:40
To: [email protected]
Subject: st: Possible bug
I am running a regression model using both quantile regression and robust
regression. In my output, robust regression reports 1 less observation than
quantile regression (which reports the right number of observations in my
sample). Is this is a feature of robust regression, am I missing something,
or is it a bug?
Thanks for your assistance,
Apostolos Ballas
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