st: RE: Relative survival - strel

[Tim Evans <Tim.Evans@wmciu.nhs.uk>]

Hi Sam,

strel is a programme for calculating relative survival, updated and maintained by the London School of Hygiene and Tropical Medicine (for Nicks information!).

I think you have to look at how strel is calculating crude survival by looking in the ado file. 

I think you could say there are two types of crude survival  - one where you simply divide number of events by the number of cases (which takes no account of survival time) - so 'Crude' crude survival, or you can have a sophisticated crude survival which takes account of the survival time or person years in the analysis and gives a weighted crude survival. This is what strel does - if you look at the code in the ado file, you will see this:

gen Crude=exp(-sum(deaths/p_years*_width))*100

This is why you don't get an exact division in the manner you suggest. An exact division would be if the survival time of all of the death cases was 2.5 years.

I think its upto you to decide which is more useful for you, but the above is common practice and if you used strs (an alternative programme in Stata) it also calculates crude survival like this.

As an aside, I don't know what data you are calculating relative survival on, but your break points seem too wide to take into account variations in underlying mortality. By having only one break point of 5 years, you are assuming that there is no change in risk of mortality between 0-5 years. Without knowing your data, I doubt this is a correct assumption.

Hope this helps

Tim


-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Sam Leary
Sent: 06 October 2011 11:07
To: statalist@hsphsun2.harvard.edu
Subject: st: Relative survival - strel

Dear all,  I am a bit of a novice at survival analysis, but am trying to 
calculate relative survival using strel.  My variables are

death_5y = 1 if died by 5 years after diagnosis, 0 if still alive at 5 years
age_diag = age at diagnosis (years)
age_death = age at death or 5 years, whichever came first (years)

I have downloaded a lifetable which has the following variables:
country calendar_year sex age rate
and I have only kept the data for the calendar year relating to the start 
date of my audit.  I have also sorted this file by age then sex.

So I have used the following commands to calculate the crude and relative 
survival at 5 years after diagnosis:

stset age_death, failure(death_5y) origin(age_diag)
strel 0 5 using "ltab_s1",mergeby(sex)

My question is, as there are 257 deaths out of a total of 468, why is the 
crude survival percentage 41.86 using strel, rather than (468-257)/468 = 
45.1?

Many thanks,

Sam.

----------------------
Dr Sam Leary
Senior Lecturer in Statistics

Working hours:
Monday, Tuesday and Thursday
7.30am - 2.30pm

Lifecourse Epidemiology and Population Oral Health Research Group
Department of Oral and Dental Sciences
Bristol Dental School
Lower Maudlin Street
Bristol
BS1 2LY
Tel 0117 342 3264
Fax 0117 342 4443
E-Mail: S.D.Leary@bristol.ac.uk
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