topic Re: Use Previous Observation Values to Calculate Current Observation in SAS Programming

Use Previous Observation Values to Calculate Current Observation

djbateman — Fri, 28 Oct 2011 12:52:29 GMT

I know that SAS can do this with PROC LIFETEST, but I want to attempt this by hand. Even though I can easily do this in Excel or R, this might be a good learning experience. I am essentially trying to reproduce the product-limit table for some survival data. Below is a look at how my data currently looks. The value in "AtRisk" is a macro variable placed in the dataset if it is the first observation.

What I want to do is calculate AtRisk(i) = AtRisk(i-1)-censored(i-1)-event(i-1). In other words, AtRisk for SurvivalTime=2 is 49-5-0=44. Then At Risk for SurvivalTime=2 is 44-0-1=43, etc.

I have tried using the RETAIN statement and the lag() function to perform this, but I cannot get either one to work. If you have any better ideas, please help me out!

Here is an example of my code that does not do the trick:

data data2;

set data1;

by _LABEL_; /* NOTE: _LABEL_ has the same value throughout the data */

if first._LABEL_ then atrisk=&atrisk.;

else atrisk=lag(atrisk);

run;

CURRENT DATA:

_NAME_	SurvivalTime	Censorred	Event	AtRisk
COUNT	1	5	0	49
COUNT	2	0	1	.
COUNT	3	0	1	.
COUNT	4	1	0	.
COUNT	5	0	1	.
...	...	...	...	...

CORRECT DATA:

SurvivalTime	Censored	Event	AtRisk
1	5	0	49
2	0	1	44
3	0	1	43
4	1	0	42
5	0	1	41
...	...	...	...

Re: Use Previous Observation Values to Calculate Current Observation

DLing — Fri, 28 Oct 2011 13:14:24 GMT

How about this?

data data2;

set data1;

retain atrisk;

if _n_ = 1 then atrisk = &atrisk;

else atrisk = atrisk - censored - lag( event );

run;

Re: Use Previous Observation Values to Calculate Current Observation

art297 — Fri, 28 Oct 2011 13:20:42 GMT

%let AtRisk=49;

data want (drop=Last:);

input _NAME_ $ SurvivalTime Censorred Event;

retain AtRisk LastAtRisk;

AtRisk=ifn(_n_ eq 1,&AtRisk,

LastAtRisk);

LastAtRisk=AtRisk-Censorred-Event;

cards;

COUNT 1 5 0

COUNT 2 0 1

COUNT 3 0 1

COUNT 4 1 0

COUNT 5 0 1

;