Re: st: comparing two linear slopes

[Steven Samuels <sjsamuels@gmail.com>]

Ashwin, I have three comments:

1. It is unlikely that hospitalizations are independent events;  
therefore analyses based on the independence assumption will be  
faulty. Non-independence could arise, for example, from: repeated  
hospitalizations of the same person; seasonal variation; serial  
correlation; clustering of events in different departments and  
institutions.

2. Trends in the proportions of admissions for the two diseases are  
affected by trends in the numbers of admissions of other diseases.

3. Formally, comparison of trends in _numbers_ of admissions for the  
two diseases can be directly studied in a data set that excludes all  
other admissions.

Perhaps a multiple time-series analyses of the two types of admissions  
will address these issues. This is an area I haven't worked in, so I  
can't begin to recommend specific Stata commands.

Steve

Steven J. Samuels
sjsamuels@gmail.com
18 Cantine's Island
Saugerties NY 12477
USA
Voice: 845-246-0774
Fax: 206-202-4783fore

On Oct 29, 2010, at 4:40 PM, Ashwin Ananthakrishnan wrote:

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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