topic Re: Power calculation in cohort study in Statistical Procedures

Power calculation in cohort study

hovliza — Tue, 02 Jan 2018 12:34:01 GMT

Hi! I am performing a prospective cohort study in an existing cohort study. The sample of participants I can use from this existing cohort is 190. I am using a Cox model to analyze the data and to get a Hazard ratio. But now I want to know what the power of the study is. How can I calculate it? I can't find it on the internet...

So I am doing a prospective survival analysis (in an existing cohort) of 190 participants, the exposure is divided either into three categories and continuous and the outcome is yes/no.

After that, I also want to calculate the power of the study when it is stratified for some covariates.

I am using SAS, so maybe there is an option in SAS for this?

I hope some of you can help me. Thanks in advance!

Re: Power calculation in cohort study

Reeza — Tue, 02 Jan 2018 16:00:26 GMT

PROC POWER.

It looks like survival analysis.

Here's an example to get you started.

http://documentation.sas.com/?docsetId=statug&docsetTarget=statug_power_examples06.htm&docsetVersion=14.3&locale=en

@hovliza wrote:

Hi! I am performing a prospective cohort study in an existing cohort study. The sample of participants I can use from this existing cohort is 190. I am using a Cox model to analyze the data and to get a Hazard ratio. But now I want to know what the power of the study is. How can I calculate it? I can't find it on the internet...

So I am doing a prospective survival analysis (in an existing cohort) of 190 participants, the exposure is divided either into three categories and continuous and the outcome is yes/no.

After that, I also want to calculate the power of the study when it is stratified for some covariates.

I am using SAS, so maybe there is an option in SAS for this?

I hope some of you can help me. Thanks in advance!

Re: Power calculation in cohort study

hovliza — Wed, 03 Jan 2018 10:44:44 GMT

Thanks for answering. When I do this:

PROC POWER;
coxreg
      hazardratio = 1.4
      stddev = 1.2
      ntotal = 190
      eventprob = 0.25
      power = .
   ;
run;

SAS says about the 'coxreg'; 180-322: Statement is not valid or it is used out of proper order. This nearly the same as in the example, what can I do? I am using SAS 9.4.. I think that is the problem? What else can I do?

Re: Power calculation in cohort study

Reeza — Wed, 03 Jan 2018 15:06:22 GMT

What version of SAS/STAT do you have? If you're not sure, you can use the following code to check.

It's possible that it's an earlier version and you don't have this feature yet 😞

proc product_status;run;

Re: Power calculation in cohort study

hovliza — Wed, 03 Jan 2018 19:40:12 GMT

I am using SAS 9.4 so I think it is not possible? What could I do to calculate the power of my survival analysis? Because Ive three groups and in PROC POWER; twosamplesurvival you only have two groups right?

Re: Power calculation in cohort study

Reeza — Wed, 03 Jan 2018 19:46:16 GMT

I would probably simulate it then.

You would run a whole bunch of simulations and determine the cutoff points. I actually coded this about 5 years ago for a clinical trial protocol and to determine the number of obs and when we could terminate early 🙂

I recall using a text book, I'll dig up the name and reference tonight, if someone else hasn't already answered this.

You can also look up Rick Wicklin's book on simulating data with SAS.