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st: diffs-diffs and sample size


From   Ana Gabriela Guerrero Serdan <ag_guerreroserdan@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: diffs-diffs and sample size
Date   Thu, 7 Feb 2008 04:30:03 -0800 (PST)

Dear all, 

I am  using the natural experiments methodology  and
calculating difference-in-difference estimators for
different cohorts of children in different treatment
areas. 

Yit=  b1 + b2 (Cit) + b3 (Tit) + b4 (CTit)+ e  

I want to see the effect of the treatment on young
cohorts and wonder if I have enough observations to
split my sample in 5 cohorts and 4 areas (3 treatment
areas + 1 control) and then run my diffs-diffs
estimation for each treatment/cohort.  

my total sample includes 14,000 children spread shown
in the following table: 

                         Cohort 
District     0	   1	2  3     4     5	Total
							
Treatment 1  152  292 703 949  892    799	3,787
Treatment 2  95  173  478  608  575   504      2,433
Treatment 3  186 403 975  1,230  1,211 1,009   5,014
Control      118  246  626  823	 745    642     3,200
							
Total	551  1,114  2,782  3,610  3,423	2,954  14,434

I wonder if I could use sampsi:
mean1=cohort0 mean2=cohort5
sampsi .0270862  -1.122313, p(0.90)  sd1(1.867671)
sd2( 1.340558)

Estimated sample size for two-sample comparison of
means

Test Ho: m1 = m2, where m1 is the mean in population 1
and m2 is the mean in population 2
Assumptions:

alpha =   0.0500  (two-sided)
power =   0.9000
m1 =  .027086
m2 = -1.12231
sd1 =  1.86767
sd2 =  1.34056
n2/n1 =     1.00

Estimated required sample sizes:

n1 =       43
n2 =       43



or I should be using the mean of cohort0 in treatment
area1 Vs mean cohort0 in control area? 

thanks

Gaby 




Gaby Guerrero Serdan 

Deparment of Economics
Royal Holloway, University of London
TW20 OEX
Egham, Surrey
England, UK
http://www.rhul.ac.uk/economics/About-Us/postgrads.html
http://www.flickr.com/photos/49939890@N00/show/

Tel: +44 7912657259


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