Programming the statistical procedures from SAS

Why no solution computed in PROC POWER

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Frequent Contributor
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Why no solution computed in PROC POWER

Dear,

Currently I am using SAS 9.4 to calculate the power with the given size and un-equal variance between 2 groups. However, when I submitted the following code, it turned out there was NO SOLUTION COMPUTED, hence I seek your help on this.

By the way, I have read the guidance regard to this test method, and found there was no wrong in the code.

The Code:

proc power;

  twosamplemeans test=diff_satt

 

  groupmeans=(1.2 0.1)/*Specify group means: grp1_mean=1.2, grp2_mean=0.1*/

  alpha=0.1          /*Specify the test significance level*/

  sides=2             /*Specify the test is two-sided*/

  groupstddevs=(1.2, 0.1)/*Specify the standard deviation for each group: grp1_std=1.2 grp2_std=0.1*/

  groupns=(10 10)        /*Specify the number of sample size in each group:grp1_N=grp2_N=10*/

  power=.                /*Request the power with above prerequisite*/

;

run;

sample size calculation.png

Thanks in advance,

Frank


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Solution
‎09-02-2014 10:28 AM
SAS Super FREQ
Posts: 3,319

Re: Why no solution computed in PROC POWER

You can use simulation to estimate the power. See the program at   Using simulation to estimate the power of a statistical test - The DO Loop but use

RejectH0 = (Probt <= 0.1);

and

method="Satterthwaite".

The estimate of the power is 0.8530, with a 95% CI of [0.8311, 0.8744].

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Respected Advisor
Posts: 2,655

Re: Why no solution computed in PROC POWER

I was able to duplicate this behavior, and was a bit surprised.  I tried two one-sided tests at alpha=0.2 and was able to get results, but am baffled by this particular behavior.  It remains until the Group 2 standard deviation gets to about 10% of the Group 1 value.  So you could say the power was greater than 0.846.

It appears that this result occurs whenever the ratio of the SD's is greater than 10.

Steve Denham

Frequent Contributor
Posts: 86

Re: Why no solution computed in PROC POWER

Dear Steve,


Thanks so much. Yes, this is another good way to present.

Frank

Solution
‎09-02-2014 10:28 AM
SAS Super FREQ
Posts: 3,319

Re: Why no solution computed in PROC POWER

You can use simulation to estimate the power. See the program at   Using simulation to estimate the power of a statistical test - The DO Loop but use

RejectH0 = (Probt <= 0.1);

and

method="Satterthwaite".

The estimate of the power is 0.8530, with a 95% CI of [0.8311, 0.8744].

Frequent Contributor
Posts: 86

Re: Why no solution computed in PROC POWER

Dear Rick,

Thank you so much, yeah. Simulation is one way to calcualte the power. However, it seems in PROC POWER, this is one bug? Hope in future this can be resolved.

Best,

Frank

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