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From |
"Carlo Lazzaro" <carlo.lazzaro@tin.it> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
st: R: comparing two linear slopes - flag: Stata 9.2/SE |

Date |
Sun, 31 Oct 2010 11:49:22 +0100 |

Ashwin may want to consider the increased number of hospitalizations between "year end" and "year start" for both the diseases under investigation (A and B) as a proxy for the increase in the number of episodes of A and B. Hence, Ashwin may want to perform a parametric ttest on the differences in hospitalizations for A and B and then test the results in terms of significance via a bootstrap p-value: *********************** code begins ************************************* drop _all set obs 100 g H_1_Start=2*uniform() g H_1_End=8*uniform() g H_2_Start=1*uniform() g H_2_End=2*uniform() g Diff_1= H_1_End- H_1_Start g Diff_2= H_2_End- H_2_Start ttest Diff_1 == Diff_2, unpaired unequal return list scalar tobs=r(t) replace Diff_1= Diff_1-r(mu_1)+(r(mu_1)+r(mu_2))/2 replace Diff_2= Diff_2-r(mu_2)+(r(mu_1)+r(mu_2))/2 bootstrap r(t), reps(10000) nodots saving(C:\Documents and Settings\carlo\Desktop\Ashwin_boot.dta, every(1) double replace): ttest Diff_1 == Diff_2, unpaired unequal use "C:\Documents and Settings\carlo\Desktop\Ashwin_boot.dta", clear g indicator =abs(_bs_1)>=abs(scalar(tobs)) sum indicator, mean di "p_bootstrap =" r(mean) *********************** code ends ************************************* HTH and Kind Regards, Carlo -----Messaggio originale----- Da: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] Per conto di Ashwin Ananthakrishnan Inviato: venerdì 29 ottobre 2010 22.40 A: statalist@hsphsun2.harvard.edu Oggetto: st: comparing two linear slopes 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 * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/ * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: comparing two linear slopes***From:*Ashwin Ananthakrishnan <ashwinna@yahoo.com>

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