st: Computing Quintiles with frequency weight

["Polloni Stefano" <stefano.polloni@umontreal.ca>]

Greetings,

I am trying to observe comsumption patterns among the different income quintiles of a population. My sample contains 10 801 observations, each with a corresponding frequency weight so as to be representative of the whole population (about 30 million people). 

I am using this command to compute my quintiles:

xtile quintile= hhinctot[fw=weight], n(5)     
(hhinctot as the income variable)

After doublechecking, I realized that some of my quintiles contained substantially less or more than 20% of the observations (while effectively taking in consideration the frequency weight - i.e. as if I duplicated myself the observations according to their frequency)

this is the result I get:

Quintile 1: 20,406%
Quintile 2: 20,552%
Quintile 3: 19,239%
Quintile 4: 22,323%
Quintile 5: 17,479%

I can get more precise results by computing the quintile myself "manually", any idea why stata gets this far from 20% ?

Thank you for your time!

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