Ok. I'm a bit lost here. I really don't understand all the steps (especially
step 2) but I'll try to do them anyway.
*1:
Like before:
The 600:
age_grp n_age_grp_s pct_age_grp_s
1 38 6.33
2 47 7.83
3 41 6.83
4 41 6.83
5 44 7.33
6 38 6.33
7 44 7.33
8 48 8.00
9 43 7.17
10 41 6.83
11 42 7.00
12 35 5.83
13 39 6.50
14 33 5.50
15 26 4.33
Total 600
And the 4975:
age_grp n_age_grp pct_age_grp Cum.
1 450 9.05 9.05
2 438 8.80 17.85
3 395 7.94 25.79
4 375 7.54 33.33
5 376 7.56 40.88
6 370 7.44 48.32
7 344 6.91 55.24
8 315 6.33 61.57
9 306 6.15 67.72
10 299 6.01 73.73
11 275 5.53 79.26
12 271 5.45 84.70
13 263 5.29 89.99
14 241 4.84 94.83
15 257 5.17 100.00
Total 4975
So weight1 is defined as:
gen weight1=.
replace weight1 = 450 / 38 if age_grp == 1
replace weight1 = 438 / 47 if age_grp == 2
replace weight1 = 395 / 41 if age_grp == 3
replace weight1 = 375 / 41 if age_grp == 4
replace weight1 = 376 / 44 if age_grp == 5
replace weight1 = 370 / 38 if age_grp == 6
replace weight1 = 344 / 44 if age_grp == 7
replace weight1 = 315 / 48 if age_grp == 8
replace weight1 = 306 / 43 if age_grp == 9
replace weight1 = 299 / 41 if age_grp == 10
replace weight1 = 275 / 42 if age_grp == 11
replace weight1 = 271 / 35 if age_grp == 12
replace weight1 = 263 / 39 if age_grp == 13
replace weight1 = 241 / 33 if age_grp == 14
replace weight1 = 257 / 26 if age_grp == 15
*2:
?????
How do I estimate
*3:
*4:
Now I generate a variable called sample which is 1 for each of the 600 and 0
for the rest of the 3743.
.tab sample
sample Freq. Percent Cum.
0 3,143 83.97 83.97
1 600 16.03 100.00
I now generate the probability of inclusion using just age and geography to
make things simple:
xi: logistic sample i.age_grp i.geo_grp
Predict p_r
*5:
gen weight2 = weight1 * (1/p_r)
*6:
Now I generate the totals for age and geography:
*age
gen pct_agex = .
replace pct_agex = 450 / 4975 if age_grp == 1
replace pct_agex = 438 / 4975 if age_grp == 2
replace pct_agex = 395 / 4975 if age_grp == 3
replace pct_agex = 375 / 4975 if age_grp == 4
replace pct_agex = 376 / 4975 if age_grp == 5
replace pct_agex = 370 / 4975 if age_grp == 6
replace pct_agex = 344 / 4975 if age_grp == 7
replace pct_agex = 315 / 4975 if age_grp == 8
replace pct_agex = 306 / 4975 if age_grp == 9
replace pct_agex = 299 / 4975 if age_grp == 10
replace pct_agex = 275 / 4975 if age_grp == 11
replace pct_agex = 271 / 4975 if age_grp == 12
replace pct_agex = 263 / 4975 if age_grp == 13
replace pct_agex = 241 / 4975 if age_grp == 14
replace pct_agex = 257 / 4975 if age_grp == 15
gen tot_agex = round(pct_agex * 10000)
replace tot_agex = tot_agex - 1 if agex ==1
*Geography
gen pct_geo =.
replace pct_geo = 2726 / 4975 if geo_gr==1
replace pct_geo = 2249 / 4975 if geo_gr==2
gen tot_geo = round(pct_geo * 10000)
* Now I rake weight2 back to the age categories & geographics
keep if sample==1
survwgt rake weight2, ///
by(age_grp geo_grp) ///
totvars(tot_agex tot_geo) ///
gen(weight3)
*7
Here I make new variables for tot_agex and tot_grp from data from the Danish
Census (_DC) like this:
*age
gen pct_agex = .
replace pct_agex_DC = (DC population total in age_grp==1) / (DC population
total) if age_grp == 1
.
.
.
.
replace pct_agex_DC = (DC population total in age_grp==15) / (DC population
total) if age_grp == 15
gen tot_agex_DC = round(pct_agex_DC * 10000)
And the same for tot_geo_DC
Then I use the rake again
survwgt rake weight3, ///
by(age_grp geo_grp) ///
totvars(tot_agex_DC tot_geo_DC) ///
gen(weight4)
svyset [pweight=weight4], strata(agex)
So to estimate ed in the general populaion I would do:
svymean ed
Is it correct?
Steven if you give me your personal details I'll include you in the
acknowledgements of the paper if you'd like.
Best regards
Kristian
-----Oprindelig meddelelse-----
Fra: [email protected]
[mailto:[email protected]] På vegne af Steven Samuels
Sendt: Monday, December 08, 2008 6:13 PM
Til: [email protected]
Emne: Re: SV: S: SV: st: Survey - raking - calibration - post stratification
- calculating weights
On Dec 8, 2008, at 2:55 AM, Kristian Wraae wrote:
> Ok, thanks.
>
> Now I understand how to do the raking procedure.
>
> I have one question though.
>
> Since I have a two step inclusion procedure wouldn't it be more
> accurate to
> rake in two steps.
>
> Example:
> I know the distribution of medication amongst the 3745 men.
>
> But the 3745 men differs from the 4975 men by being slightly
> younger and we
> know that the older you get the more medicin do you get. That also
> goes for
> physical activity and smoking.
>
> So if I calculate the expected prevalences amongst the 4975 (in
> order to
> rake the 600) from the 3750 I risk making a mistake
> (underestimating the
> prevalences in the baclground population). I guess should be
> calculating the
> all prevalences from the 4975, but I don't those data.
>
> So wouldn't it be more correct to:
>
> 1. Rake the 3750 so they match the 4975 on age and geography.
>
> 2. Calculate all the expected prevalences on age, medication, smoking,
> physical activity ect from the now raked 3750 (as we would expect
> them to be
> had we had a 100% response rate).
>
> 3. Use these prevalences to rake the 600 as you showed me?
Your concern is a good one, Kristian. However, the solution you
propose is ad-hoc with no real theoretical justification. I've tried
some complicated raking in the past, but I have never seen a
reference to the method you propose. You have much questionnaire
information on too many informative variables; raking can use only a
small part of it. There is a standard approach to this problem:
model the probability of participating in the phone interview. I
suggest you consult the text "Statistical Analysis with Missing Data"
by Little & Rubin, especially Chapters 3 & 13. In the parlance of
that book, you must assume that data are "Missing at Random". This
means that the probability of having a phone interview depends
completely on characteristics known from the mail questionnaire or
the census.
Here are the steps:
1. Estimate weight1 = N_i/n_i as before for the 15 age groups.
2. You can use this weight on the second phase sample of 3,750 to
estimate various properties of the population known such as
proportions in categories of medication, physical activity smoking.
These may be of interest in themselves.
3. Instead of raking, use -logistic- or -logit- (not the survey
versions) on the 3,750 men to predict who participated in the
telephone interview. Consider as covariates: age, geography,
medication, physical activity, smoking and any others that might be
of use.
4. Generate the predicted probability of participating in the
telephone interview. Call this p_r. Your goal is to get a good
prediction, so compute ROC curves, if possible. (I don't recall if
Stata 8 has the -lroc- command.)
5. For the 600 men in the telephone survey, compute: weight2 =
(weight1) x (1/p_r).
6. Rake weight2 back to the age categories & geographic categories
of the 5,000 men. Call the result "weight3".
7. Finally rake weight3 to the Danish Census age/geographical
breakdowns: Call it "weight4".
7. Use this as your final analysis weight for -svymean-.
You are a long way from the simplicity of Stas's earlier suggestion
to use "weight1" on your data. Standard errors that you compute will
be under-estimated, because they do not account for the uncertainty
in the estimating "weight3", and you must state this in your report.
If you wish to compute the proper standard errors, you must, I think,
bootstrap the process starting no later than Step 3. This is the
price for using the complex sampling design.
-Steve
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