Standardized Differences binary categorical variable

lisahoward — Thu, 02 Apr 2015 06:42:03 GMT

I have been asked to calculate the Standardized difference for continuous and categorical variables. I understand how to do it for the continuous variables but am unsure how to do it for the binary categorical variables. This is the code I use for the continuous variables where x and y are the means of the two variables for the two groups Y =treated X = Untreated :

I use proc means to get the mean for the continuous variable in the untreated and treated groups first and then run the following code:

data Stan_Diff;

set Cohortfile;

if Var_Type='Cont' then do;

length stddiff diff 8;

format stddiff 6.2 diff 8.4;

denomSD = sqrt((y_&varname_Treat**2 + x_&varname_UnTrt**2)/2);

diff = x_&varname_Treat - y_&varname_UnTrt;

if denomSD gt 0 then do;

stddiff = 100*(nx_&varname_Treat - ny_&varname_UnTrt)/denomSD;

end;

else stddiff = .;

end;

run;

For the Binary variables I am using proc freq to get total counts and percentages not proc means. Should I be using proc means. I am unsure how to get the SD without creating a mean value first.

My data set looks like this: For the continous variables Age and Medications I use proc means to produce a mean.

For the catagorical variables Diabetes Asthma etc I use proc freq to generate the count and percentage for my output table. So this is where I get stuck as with out a mean how do i produce the SD?

Patient	Exposure	Age	Medications	Diabetes	Asthma	Smoking	Osteo	Fracture
1	1	50	6	1	0	1	0	1
2	1	54	4	0	0	1	0	1
3	0	42	11	0	1	1	1	1
4	1	35	0	1	1	0	1	0
5	0	70	9	1	0	1	0	1
6	0	55	12	1	1	0	1	0

The table I am trying to create looks like this:

		Before propensity score match
		(N=xxx)
Baseline Characteristics		Treated	Untreated	Standardized Difference
Baseline Characteristics		(N=xx)	(N=xx)	Standardized Difference
Demographics
	Age – Mean	xx.x (xx.x)	xx.x (xx.x)	x.xxx
	Number of unique medications – Mean	xx.x (xx.x)	xx.x (xx.x)	x.xxx
Baseline Comorbidities
	Diabetes– count (%)	xx.x (xx.x)	xx.x (xx.x)	x.xxx
	Asthma – count (%)	xx.x (xx.x)	xx.x (xx.x)	x.xxx
	Smoking – count (%)	xx.x (xx.x)	xx.x (xx.x)	x.xxx
	Osteoporosis – count (%)	xx.x (xx.x)	xx.x (xx.x)	x.xxx
	Fracture – count (%)	xx.x (xx.x)	xx.x (xx.x)	x.xxx

Many thanks in advance,

Lisa.

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Standardized Differences binary categorical variable