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st: dropping even observations

From   "Nick Cox" <>
To   <>
Subject   st: dropping even observations
Date   Thu, 5 Oct 2006 10:45:17 +0100

The following question and reply arose privately. 
I have been asked this various times before, so the 
discussion should be of some wider interest. 


Question: Do you have an expression or do file I could 
use or adapt to drop even rows (_n = 2,4,6 etc) or 
dropping every other row?  


Sure. The remainder on dividing integers 
by 2 is either 1 or 0 depending on whether 
those integers are odd or even. In Stata 
with observation numbers _n this remainder
is simply 


Logical negation ! flips 0 and 1 the 
other way round. 

Thus try 

sysuse auto, clear 
list mpg if mod(_n,2) 
list mpg if !mod(_n,2)

and so forth. 

Note that 

1. you do not need to create any extra 

2. the technique generalises easily
to related problems: e.g. every 5th 
observation is selected by 

if !mod(_n,5) 

Your query adds support to my longstanding
view that there are useful functions that
people persistently overlook, although 
their usefulness can be blindingly obvious
once pointed out. Sooner or later I will
write a Stata Journal Tip on -mod()-.


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