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st: comparing two linear slopes


From   Ashwin Ananthakrishnan <ashwinna@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: comparing two linear slopes
Date   Fri, 29 Oct 2010 13:40:16 -0700 (PDT)

Hi, 

I'm examining the rate of change of two proportions over time. I'd be grateful if someone could help me figure this out: 

I'm looking at the proportional increase in two specific disease (as a proportion of all hospitalizations) over time and need to see if the rate of increase for one disease is statistically greater than the rate of increase for disease 2. 

I have the sum totals for the numerators (i.e disease 1 or disease 2) and denominators (total hospitalization 1 and total hospitalizations 2) for the start and the end year (the relationship is linear for the intervening years - that's been looked at). 

Is there a way to compare these two trends statistically to say that the 4-fold increase for disease 1 is significant greater than the 2-fold increase for disease 2? 

Should i calculate the annual percent change for each disease and then use the two-sample t test? How i get the Std dev for the annual percent change? 

Thanks, 

Ashwin


      
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