topic Re: Combine items (5 point likert) into one scale in SAS Procedures

Combine items (5 point likert) into one scale

jgraupner — Tue, 18 Feb 2020 19:47:05 GMT

I've used an externally validated, 17-item self-efficacy scale at pre/post an intervention. Answers range from 1 to 5 on a Likert scale. To analyze the different between pre/post, I've been told to combine all 17 items into one item and then do a t-test (or rank sums since ordinal?).

My question is what is meant here by "combine" and how do I go about doing this in SAS?

data looks like this:

Survey id pre/post item1 item2 item3 .......... item 17

1 pre 4 3 3 4

1 post 5 5 4 5

2 pre 3 4 5 3

2 post 4 4 5 4

***

20 pre 3 4 5 4

20 post 5 5 5 5

Re: Combine items (5 point likert) into one scale

Reeza — Tue, 18 Feb 2020 19:51:42 GMT

You need to ask the person who told you to 'combine them' what they meant. Depending on your data there may be a common composite score where you make sure all are aligned and sum them or average them or have to weight some in certain ways. Once you understand the methodology of combining them the code is quite simple. Figuring out what to code is always the hard part but this isn't really a question for us.

Re: Combine items (5 point likert) into one scale

novinosrin — Tue, 18 Feb 2020 20:01:09 GMT

Very neat.

For all I know from my short experience in industry so far:

1. There is a disconnect between Industry and Academia

2. There is a disconnect between Business & Technology

3. There is a disconnect between Business & process

Overall, it appears the show is run by sales, only sales and more sales, everything else is compliance.

Re: Combine items (5 point likert) into one scale

Reeza — Tue, 18 Feb 2020 20:08:26 GMT

@novinosrin wrote:

Very neat.

For all I know from my short experience in industry so far:

1. There is a disconnect between Industry and Academia

2. There is a disconnect between Business & Technology

3. There is a disconnect between Business & process

Overall, it appears the show is run by sales, only sales and more sales, everything else is compliance.

1. Yes

2. Depends on org

3. Depends on org

Unfortunately, in my experience it takes a really good data science/analytics manager to manage these relationships and have the correct skill set to cross all three of these areas. Not many people have the ability to cross all three.

Re: Combine items (5 point likert) into one scale

jgraupner — Tue, 18 Feb 2020 21:05:34 GMT

I went back to the email and had gotten the explanation wrong. They said "add the 17 items into 1 scale and compare the mean total scores between the pre and posttest." So it looks like I should sum up the items and compare means. Does this mean I create a new "sum" variable? Thanks.

Re: Combine items (5 point likert) into one scale

Reeza — Tue, 18 Feb 2020 21:06:40 GMT

Yes, you add up all 17 into one score and then you can use PROC TTEST with a paired t-test most likely unless you were planning to run more complicated model via GLM or something else.

Re: Combine items (5 point likert) into one scale

jgraupner — Tue, 18 Feb 2020 22:29:25 GMT

Great, thanks. Do I use the SUM function? Do i have to use a class statement?

I'm trying to sum (item1, item2, item3...item17) but it doesn't appear to be the right thing to do.

Re: Combine items (5 point likert) into one scale

Reeza — Tue, 18 Feb 2020 22:33:19 GMT

That's exactly the right thing to do. Show your code if you think it's wrong somehow.

Re: Combine items (5 point likert) into one scale

jgraupner — Tue, 18 Feb 2020 23:03:27 GMT

If I want to do a paired t-test but my data (including new summed items variable) is organized like above (with pre=1, post=2), do i need to create new variables for a pre_sum_item and post_sum item?

so that I can do something like this?:

 proc ttest data=response sides=2 alpha=0.05 h0=0;
	title "Paired sample t-test example";
	paired pre_sum_item * post_sum_item;
   run;

if so, how? Thanks very much for you time!

Re: Combine items (5 point likert) into one scale

Reeza — Wed, 19 Feb 2020 02:25:59 GMT

Transposing data tutorials:
Long to Wide:
https://stats.idre.ucla.edu/sas/modules/how-to-reshape-data-long-to-wide-using-proc-transpose/

https://stats.idre.ucla.edu/sas/modules/reshaping-data-long-to-wide-using-the-data-step/