Computing a new variable in SPSS

So here we are in SPSS

I have created kind of a

a junk data set

just to help illustrate

how we can compute variables

or compute new variables

using the values from existing ones.

This is a pretty straight forward process,

it looks a little bit different

than things that we've done before

or other ways that we are going

to be using an SPSS in this course.

So it might look a little bit different,

but it is pretty straight forward

and pretty easy

once you know how to do this.

So the first thing that we'll do

is to go to Transform and then

the first option we have there

is Compute Variable. So click on that.

We get this new window that pops up

The first thing that we really want

to look at is in the top left and

and that's this target variable window.

Basically what the computer

is asking you

is to create a name for the new variable

that you are going to be computing.

This name needs to be unique,

it's one that can't have spaces in it

and it should avoid special characters.

But

in this case, what I like

to do is include some sort of piece

of information in there

in the naming convention I use

that helps me know what this is

just by looking at it.

So what we're going to be doing

is we are going to be summing items

one, two, three, four, five, six together,

adding those together.

So an easy way to do that,

is just to put sum of items.

That way when I look at this new variable

once it's generated

I know that that's the variable

that I really want to be working with.

Or running my tests on because that

that contains the information

from the other ones.

You can click on type and label.

The label is the more

drawn out explanation,

that's the one you can include character

additional characters or spaces

and special characters.

In our case,

I'm going to put this is the sum

of items

one through six.

And that way, I know that

that is going to a

give me a more

drawn out explanation of what

this variable includes.

We know this is

going to be uneric - numeric - I'm sorry,

so click Continue.

The next thing we are going to

look at is this numeric expression box

Um we're actually come back

to this in just a minute.

But this is where you're going to be doing

most of the work

when you're computing a variable.

What you want to go to next

is this function group area.

So I'm going to click on All

and you notice right below it

we've got these functions

and special variables

and this pop up when I clicked on all.

This has a whole bunch

of different pre-existing functions,

so you don't need to reinvent the wheel.

You can basically use these pre-existing

functions to help you compute

this new variable.

Um, I'm going to go down to sum

and these are in alphabetical order

so it's pretty easy to find.

Here we have Sum and when I click on Sum,

you'll notice that this box to the left

populated with some text.

This box is actually pretty useful.

So the first thing it shows you

is an example of what this expression

is going to be.

I know expression

sounds likes a really fancy term,

if you've ever used a formula in Excel

it's basically that.

It's telling the computer

what it needs to do.

In this case, just like Excel,

we've got the word sum,

which tells the computer it's going

to be summing different values together.

And then in parenthesis,

immediately after that,

we have different variables

that are separated by a comma

and that's what tells the computer

to add x, y, and z things together

to create a new variable

or compute a new variable.

This also provides additional

extra content here, but we don't need to

worry about that. I've found this

is useful if you are stuck or something

isn't working right.

So we've got some selected,

we're going to be summing the items

of one, two, three, four, five, and six

together. We've got that selected.

So the next thing you do

is click this up arrow

which basically takes that pre-existing function

and kicks it up to this

numeric expression box.

The next thing that we're going to do,

we've already told the computer

that we're going to sum

different variables together,

We need to tell it which variables

to sum together.

So the easy way to do this

is to click on the first item

we want to include and you'll notice that

this arrow switched sides

from pointing to the left to pointing

to the right, click over one,

and we've already moved over item one.

What you need to be careful of

is that you always have a comma

after each variable except for the last one.

So a comma is already here,

but what I usually do is I just hit a comma

I go to item two, kick that over,

comma, item three, kick that over,

comma, item four, kick that over,

comma, item five, kick that over,

comma, item six, kick that over.

Now we know that item six

is the last one we have,

the last variable we're including.

So what' we're going to do is just delete

out that question mark, we're going to

delete out that last comma and

then if we read through this real quick

just to make sure we don't have any problems

we've got creating sum of items,

this is going to be the sum of item one,

two, three, four, five, and six

and what we're going to do now is click OK

An output window pops up and basically

tells you what the computer did

it computed the sums, which was the sum

of items one, two, three, four, five, and six

it gives us the variable label.

It gives us that descriptive text I put in

I'm just going to minimize this window

and if you look back here, you might

notice that there is this new column,

this new variable, this sum of items,

that wasn't there when we started

and that's because SPSS computed that

and if we add these together

if you add each row

you've got one two one three five one

that adds to thirteen.

Five one two three five one

adds to seventeen.

So it's done that math for us, you didn't

have to manually do it.

I know yes it would be pretty easy

to manually do this for ten cases

but if you have a data set

that has five hundred cases

that would be very time-consuming to do.

And you can see, that this kind of function,

of computing this variable, can save you

a lot of time. It's also more accurate

It's going to take any sort of error

out of the process, error out of the

equation. And we know that, often,

when we do things by hand,

we can introduce error into things.

So we want to avoid that.

That's one way to compute a variable.

I'm going to show you one other way

a different type of variable

that we can compute.

I'm going to go back to transform

I'm going to go back to compute variable

I'm actually going to clear this out

we're done with sum of items

we don't need to worry about it

we'renot going to use the same expression

so we click reset in the bottom here

so what I want to do is create the mean

of the different items that we have.

Sometimes if you are working

with a pre-existing scale

it might tell you to add all the items

together and then that's what represents

that theoretical concept or that measure.

Sometimes it might tell you to use the mean

so what I want to do is compute

both of these things. because they both

might be helpful to me.

So what I'm going to do,

is I'm going to type in mean of items

and again, that very quickly, let's me

differentiate between sum of items

and mean of items, type of label

again, I'm just going to put mean of items

one through six

Click continue

Just like we did before

click on all and then

we're going to go down to mean.

Here's mean.

And when I click on that, again,

it gives me that sample text here,

this is very similar to what we did before

I'm going to click up

and we're going to kick over one

comma two comma three comma four

comma five and then six.

I'm making sure to include this last one

this sum of items

and then just like last time,

we're going to delete out that question mark

that last comma

we're going to read through it

really quickly just to avoid an error

we've got a new variable, the mean of item

one two three four five and six

I'm going to click OK

SPSS again showed me the syntax of kind of

what it did. I'm going to minimize that.

And then here we have the mean of items.

And if we really quickly look at this

I'm gonna pick this one for example,

we've got one one three three one two

and a mean on 1.83 would just eyeballing it

kind of make sense.

If we look at this one, 3.67

Two three five four five three

that would roughly make sense

without having to do the math of it

but this is computed the average score

the mean score for all six of those items

and that is very quickly how to compute

a new variable in STSS

This is particularly useful when

you have collected data using a pre-existing

measure and that measure has instructed you

to compute the sum of items

and run your tests on that.

Or maybe the mean of items

and run your tests on that.

But this is a useful way of understanding

how to create new variables

using existing ones in SPSS.