sas >> appropriate contrasts for 1-way anova with unequal sample sizes

by Jackson_AIG » Thu, 09 Dec 2004 00:03:31 GMT

Unequal sample sizes complicate the assignment of coefficients for
contrasts. I was taught, and some textbooks still teach, to weight the
coefficients using the sample sizes. I have not found any mention of this
in the SAS literature I have available to me, and in fact as far as I can
discover, it is impossible to use coefficients that do not sum to zero (in
CONTRAST).

Has anyone found a way to trick SAS into allowing the correct
coefficients? Or is my statistical lore terribly out-of-date?

Thanks in advance,

Tom Martin
Asst. Prof. of Aquatic Ecology
Biology Dept.
Western Carolina University

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4. Sample size estimation for ANOVA with repeated measures

5. 2 way ANOVA sample size calc

I have 3 treatment groups and 2 genders
I am planning to have
1/an equal N in each of the 6 subgroups
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mean(treatmentgroup1)-mean(treatmentgroup2)=1
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3/variance=25
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What sample size do I need?

I used proc glmpower (and obtained N=4344!). I would like to get a
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data mylib.notespb;
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   11      1	1
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;
run;



PROC GLMPOWER DATA=mylib.notespb;
CLASS Treat Gender;
MODEL DepVar= Treat Gender;
POWER STDDEV=10
ALPHA=.05
NTOTAL=.
POWER=.8;
RUN;

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7. SurveySelect not selecting where sample size exceeds sampling

Here is the sampling code (slightly anonymised).

Proc SurveySelect Data = YOURLIB.YOURDATA
                   Out = DATA.SAMPLE
                Method = SRS   N = 1000;
  Strata PRODUCT;
Run;


Due to the size of the input data table, the procedure was interrupted
through the break command.

Some thirty or so product values had been sampled, although most
produced messages similar to the following.

ERROR: The sample size, 1000, is larger than the number of sampling
units, 181.
NOTE: The above message was for the following stratum:
      PRODUCT=ABCDE.

However, a table was produced and used to verify subsequent steps that
matched analytical values.  The following log entry reflects the
completion.

WARNING: The data set DATA.SAMPLE may be incomplete.  When this step was
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When the code was run to completion (albeit on another machine), the
WARNING message also indicated that the output table was not created.
In both cases, the output table did not exist prior to the step being
run.


What did I want??????

1000 records selected randomly from each stratum.

Where the stratum had a population less than 1000, select all the
records.


In the first run, this appeared to happen.

In the second and complete run, the errors prevented any table being
created.

I can't find anything in the FM to indicate there is an option I should
set to select the whole population where the sample cannot be met, but I
may have missed something.

Possible confounding factors include that they are different machines
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the outcome.  Also that the behaviour of creating the output data set is
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Please don't suggest I hard code a data step solution.  One was proposed
but for the size of the data it resulted in a lot of time doing what a
single procedure was expected to do in a single data pass.

Kind regards

David

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8. Help with ANOVA contrasts