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# Re: st: comparing two linear slopes

 From Steven Samuels <[email protected]> To [email protected] Subject Re: st: comparing two linear slopes Date Sun, 31 Oct 2010 09:22:28 -0400

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

Steven J. Samuels
[email protected]
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,

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

Ashwin

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