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st: Looping Enquiry


From   George Bouliotis <g.bouliotis@bham.ac.uk>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: Looping Enquiry
Date   Thu, 29 Sep 2011 14:22:01 +0100

Dear Stata users,

Although an old Stata user, currently I am doing my first steps in programming. 

One of the parts in my programme tries (unsuccessfully) to replicate the column RESULT below. The difficulty is in how to loop a sequential product as, for instance: observ4= obs4 X obs3 (lag1) X obs2 (lag2)  X obs1 (lag1).

I tried some loops with "forvalue" but none was successful. I would appreciate any help with this.  

Thank you for your time
George 


Dr. George Bouliotis

Research Fellow in Biostatistics
MRC-MHTMR (Midland Hub for Trials Methodology Research) 
University of Birmingham


#####################################
set more off
clear
input    time   group   var  RESULT
          30       1        0  -0.958
          57       0        0  -0.916
          58       0        0  -0.834
          67       0        0  -0.874
          74       0        0  -0.792
          79       0        0  -0.750
          79       1        1   0.125
          82       1        1   0.125
          89       0        0  -0.706
          95       1        0  -0.662
          98       0        0  -0.678
         101       0        0  -0.574
         104       0        0  -0.532
         110       0        0  -0.448
         118       0        0  -0.444
end


list , clean 

//Generating Ssize variable
egen ssize=seq() if var==0,from(47) to(1) 
replace ssize=ssize[_n-1] if ssize==.
list, noobs clean


//Generating product variable
gen  product= (ssize/(ssize+1))  


//Generating Score variable (PRODUCT)
gen  score= 1-(2*product) in 1/1 if var==0 
// for the first observation only 

//**REPLACEMENT A: when var==0
replace  score= 1-(2*(product*product[_n-1]))  if var==0 & score==. 
// fine for the second obs only  (correct formula for when var=0)

//**REPLACEMENT B: when var==1
replace  i1= 1-1*(score*score[_n-1])  if var==1 
// fine for the second observ only (correct formula for when var=1) 
// but instead of [_n-1] I need a loop for [_n-`n(lagged)'] with "forvalue" command?

list, clean noobs



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