st: bootstrap program problem

[Fred Wolfe <fwolfe@arthritis-research.org>]

I have written a program to calculate differences between categories using 
bootstrap.

. ndbdifference sex ,by(ra) seed(1) reps(25)

Difference between ra categories                Number of obs      =     40931
                                                Replications       =        25

------------------------------------------------------------------------------
             |   Observed   Bootstrap                         Normal-based
             |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
       sex_b |  -.0353085   .0045013    -7.84   0.000    -.0441309   -.0264861
------------------------------------------------------------------------------

The program works as directed.

The code is:


program define ndbdifference
   version 9.0
   syntax varlist [if] [in] ,by(varname) [seed(real -1) Reverse reps(real 
100)]
   marksample touse, novarlist
   if "`seed'" != "-1" {
      set seed `seed'
   }
   foreach var of varlist `varlist' {
      preserve
      qui keep if `touse'
      keep `var' `by'
      qui keep if !mi(`var',`by')
      bootstrap `"`var'_b"'=r(dif), ///
      reps(`reps') nowarn nodots nolegend  title(Difference between `by' 
categories)  ///
      : difference `var',by(`by') `reverse'
      restore
   }

end

However, when I add -bca- to this line:
      bca : difference `var',by(`by') `reverse'

The Stata program crashes with this error:

    - if `"`bca'"' != "" {
    = if `"bca"' != "" {
    - forvalues i = 1/`K' {
    = forvalues i = 1/1 {
    - local name : word `i' of `names'
    = local name : word 1 of  sex_b
    - local exp_list `exp_list' `name'=(`exp`i'')
    = local exp_list `"sex_b"' =r(dif) sex_b=(r(dif))
invalid syntax

I don't understand this error, as I am not sure what is correct. However, 
if I charge the bootstrap line to:

      bootstrap `var'_b=r(dif), ///

The program runs correctly until I add -bca- again.

This time it gives the following error:

            = di as err "name sex_b already specified"
name sex_b already specified

in the section for bca.

I wonder if anyone can point me in the right direction toward understanding 
this error.

Thanks,

Fred

Fred Wolfe
National Data Bank for Rheumatic Diseases
Wichita, Kansas
Tel (316) 263-2125     Fax (316) 263-0761
fwolfe@arthritis-research.org


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