Shuffle One Variable Within Group

Question

This question is an extension of the excellent answer provided by Robert Picard here: How to Randomly Assign to Groups of Different Sizes

We have this dataset, which is the same as in the previous question, but adds the year variable:

sysuse census, clear
keep state region pop
order state pop region
decode region, gen(reg)
replace reg="NCntrl" if reg=="N Cntrl"
drop region
gen year=20 
replace year=30 if _n>15
replace year=40 if _n>35

If I just wanted to re-randomly assign reg's across all observations (without regard to group), I could implement the answer to the previous post:

tempfile orig
save `orig'
keep reg
rename reg reg_new
set seed 234
gen double u = runiform()
sort u reg_new
merge 1:1 _n  using `orig', nogen

How would the code be modified so that reg is shuffled, but only within year? For example, there are 15 observations where year==20. These observations should be shuffled separately than the other years.

Nick Cox Nick Cox · Accepted Answer · 2018-02-20T16:20:09

Shuffling one variable doesn't require any file choreography. This can probably be shortened:

sysuse auto, clear 
set seed 2803 

gen double shuffle = runiform() 

* example 1 
sort shuffle 
gen long which = _n 
sort mpg 
gen mpg_new = mpg[which] 
list which mpg* 

* example 2 
bysort foreign (shuffle) : gen long which2 = _n 
bysort foreign (mpg) : gen mpg2 = mpg[which2] 
list which2 mpg mpg2, sepby(foreign)

All that said, I think sample does this so long as you specify the same sample size as the number in the dataset. It's overkill because you get all the variables.

Shuffle One Variable Within Group

1 Answers