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How do I bootstrap a vector of results?

Note: This FAQ has been updated for Stata 14. Because bootstrap is based on random draws, results are different on previous versions because of the new 64-bit Mersenne Twister pseudorandom numbers.

Question:

    . bootstrap (stat1[1]) (stat1[2]) (stat1[3]) (stat1[4]) ///
      (stat1[5]) (stat2[1]) (stat2[2]): command ...

Answer:

The bootstrap command understands _b to mean all elements in the e(b) vector (coefficients vector posted by estimation commands). For example, you can now easily bootstrap all the coefficients from a regression:

. bootstrap  _b: regress mpg weight length ...

To take advantage of this syntax, you will have to modify your program so that it is an e-class command that posts the values of interest into e(b) instead of placing them in variables. Then, you can do something like

. bootstrap _b, reps(100): command

Here is an example that posts the vector (1,2,3,4) to e(b):

 capture program drop myepost
 program myepost, eclass
 version 13.0
 tempname bb
 matrix `bb' = 1,2,3,4
 ereturn post `bb' 
 end

 myepost
 matrix list e(b)

Here is a log of the result:

. myepost
        
. matrix list e(b)
        
 e(b)[1,4]
     c1  c2  c3  c4
 y1   1   2   3   4

Now you can use the above idea to pass a vector of results to bootstrap. To see that the method is working, you can pass the coefficients of a regression:

 capture program drop myreg
 program myreg, eclass
     version 13.0
     tempname bb
     quietly regress mpg turn
     matrix `bb'=e(b)
     ereturn post `bb'
     ereturn local cmd="bootstrap"
 end

 clear
 sysuse auto
 set seed 12345
 bootstrap _b, reps(50) nowarn: myreg 
 set seed 12345
 bootstrap _b, reps(50): regress mpg turn

Here is a log of the result:

. bootstrap _b, reps(50) nowarn: myreg
(running myreg on estimation sample)

Bootstrap replications (50)


   
 
   
 1 
   
 
   
 2 
   
 
   
 3 
   
 
   
 4 
   
 
   
 5 
 
 
..................................................    50

Bootstrap results                                           Number of obs = 74
                                                            Replications  = 50



             
 
   Observed   Bootstrap                         Normal-based 
             
 
 coefficient  std. err.      z    P>|z|     [95% conf. interval]
 
 
 
        turn 
 
  -.9457877   .1083031    -8.73   0.000    -1.158058   -.7335175
       _cons 
 
    58.7965    4.67459    12.58   0.000     49.63448    67.95853

 
. set seed 12345
 
.  bootstrap _b, reps(50): regress mpg turn
(running regress on estimation sample)

Bootstrap replications (50)


   
 
   
 1 
   
 
   
 2 
   
 
   
 3 
   
 
   
 4 
   
 
   
 5 
 
..................................................    50

Linear regression                                       Number of obs =     74
                                                        Replications  =     50
                                                        Wald chi2(1)  =  76.26
                                                        Prob > chi2   = 0.0000
                                                        R-squared     = 0.5172
                                                        Adj R-squared = 0.5105
                                                        Root MSE      = 4.0477



             
 
   Observed   Bootstrap                         Normal-based
         mpg 
 
 coefficient  std. err.      z    P>|z|     [95% conf. interval]
 
 
 
        turn 
 
  -.9457877   .1083031    -8.73   0.000    -1.158058   -.7335175
       _cons 
 
    58.7965    4.67459    12.58   0.000     49.63448    67.95853
 

There is one difference between the first program myepost and myreg: myreg has also saved e(cmd)="bootstrap". This is necessary so that bootstrap knows how it is to display the results. When you bootstrap an official Stata estimation command, bootstrap uses the estimation command's replay feature to display the coefficient table. This will show the bootstrapped standard errors since the bootstrapped covariance matrix is posted in e(V) by bootstrap. Since the command myreg doesn’t have a replay feature, you need to use bootstrap to display the results. You can do this by setting e(cmd)="bootstrap".