Bookmark and Share

Notice: On March 31, it was announced that Statalist is moving from an email list to a forum. The old list will shut down on April 23, and its replacement, statalist.org is already up and running.


[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

st: correct confidence intervals of -mean- ?


From   Dirk Enzmann <dirk.enzmann@uni-hamburg.de>
To   statalist@hsphsun2.harvard.edu
Subject   st: correct confidence intervals of -mean- ?
Date   Sat, 06 Mar 2010 13:47:41 +0100

Very carefully I want to ask: Are the confidence intervals given by -mean- really correct?

Below I compare the results of -mean- with the results of a different procedure:

* --------------------------------------
sysuse auto, clear
mean price, over(rep78)

local df = e(df_r)
display "degrees of freedom = n-1 = `df'"
* Note that here df is the number of the total sample - 1!

* alternative route:
collapse (mean) mprice=price (sd) sdprice=price (count) nprice=price if (rep78 < .), by(rep78)
* calculate the confidence intervals the -mean- way:
gen ci95la = mprice - invttail(`df',.025)*sdprice/sqrt(nprice)
gen ci95ua = mprice + invttail(`df',.025)*sdprice/sqrt(nprice)
* calculate the confidence intervals a different way:
gen ci95lb = mprice - invttail(nprice-1,.025)*sdprice/sqrt(nprice)
gen ci95ub = mprice + invttail(nprice-1,.025)*sdprice/sqrt(nprice)
* compare both sets of confidence intervals (..a vs ..b):
list
* --------------------------------------

My question: Which procedure is correct?

*************************************************
Dr. Dirk Enzmann
Institute of Criminal Sciences
Dept. of Criminology
Schlueterstr. 28
D-20146 Hamburg
Germany

phone: +49-(0)40-42838.7498 (office)
       +49-(0)40-42838.4591 (Mrs Billon)
fax:   +49-(0)40-42838.2344
email: dirk.enzmann@uni-hamburg.de
www: http://www2.jura.uni-hamburg.de/instkrim/kriminologie/Mitarbeiter/Enzmann/Enzmann.html
*************************************************
*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/


© Copyright 1996–2014 StataCorp LP   |   Terms of use   |   Privacy   |   Contact us   |   Site index