topic Re: Frequency comparison in SAS Programming

Frequency comparison

aska_ujita — Wed, 22 Apr 2020 18:15:30 GMT

Hi there, I have a data with two treatments (treated and control group).

I have behavioral class (Adequate, Calm, Very calm, Restless and Very restless) and its percentage in each group .

I did the proc freq and would like to compare the percentage frequency between group in each score.

My table is like this:

Class	Score	Treatment	Treatment	P-value
		TREATED (%)	CONTROL (%)
Adequate	16.9 to 21.2	0.00	11.11
Restless	21.3 to 25.6	0.00	6.35
Calm	12.5 to 16.8	5.56	9.52
Very calm	≤ 12.4	94.44	73.02
Very restless	≥ 25.7	0,00	0,00

I would like to calculate the P-value.

Is there a way to do that?

Thank you.

Re: Frequency comparison

ballardw — Wed, 22 Apr 2020 18:56:54 GMT

p-value implies a test. What test do you want to perform?

Re: Frequency comparison

aska_ujita — Wed, 22 Apr 2020 19:02:16 GMT

Hi Ballardw, thank you for helping me.

I would like to perform a t-test, thank you!

Re: Frequency comparison

FreelanceReinh — Sun, 26 Apr 2020 15:30:01 GMT

@aska_ujita wrote:

I would like to perform a t-test, thank you!

Hi @aska_ujita,

A two-sample t-test (see PROC TTEST documentation) compares the means of a continuous variable in two groups. So, not the percentages of the classified scores, but the original scores might be amenable to this test (especially if their distributions in the two treatment groups are approximately normal with similar variances). As a minimum for computing the test statistic you would need N (number of non-missing values), mean and standard deviation (or variance) per treatment group.

If only the classified scores are available, you can use a chi-square test or (in particular for small samples) Fisher's exact test for a comparison of the two frequency distributions (test of homogeneity). Both are available in PROC FREQ. I would not report p-values for comparisons of individual categories (unless these comparisons were pre-planned, with precautions for multiple testing). To assess the contribution of each category to the overall chi-square statistic, you can use the CELLCHI2 option of the TABLES statement, as shown in the code below:

/* Create sample data for demonstration */

data have;
do trt='treated','control';
  do class=1 to 5;
    input n @@;
    output;
  end;
end;
cards;
238 14 0 0 0
184 24 28 16 0
;

proc freq data=have;
weight n;
tables trt*class / chisq cellchi2 fisher;
run;

If you hypothesize a location shift in the distribution of scores in the treated vs. control group, a Wilcoxon rank-sum test would be a more specific approach. This test is available in PROC NPAR1WAY. Here's a code example for the above input dataset HAVE:

proc npar1way data=have wilcoxon plots=none;
class trt;
freq n;
var class;
run;

Re: Frequency comparison

PaigeMiller — Sun, 26 Apr 2020 15:31:45 GMT

@aska_ujita wrote:

Hi Ballardw, thank you for helping me.

I would like to perform a t-test, thank you!

I think @ballardw wanted you to state exactly what hypothesis you want to test.