Computing a new variable in SPSS

This video will show you how to compute the sum of multiple existing variables and how to compute the mean of multiple existing variables.

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 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 uh 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

1, 2, 3, 4, 5, 6 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

1 through 6.

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 popped 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 helpful if

you are stuck or

something isn't working right.

So we've got sum selected,

we're going to be summing the items

of 1, 2, 3, 4, 5, and 6 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 it 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 that you 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 2,

kick that over,

comma,

item 3,

kick that over,

comma,

item 4,

kick that over,

comma,

item 5,

kick that over,

comma,

item 6,

kick that over.

Now we know that item 6

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 1,

2, 3, 4, 5, and 6

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

An output window pops up and it basically

tells you what the computer did

it computed the sum of items,

which was the sum

of item 1, 2, 3, 4, 5, and 6

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 we look back here,

you might notice that there is this

new column,

this new variable, that 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 1-2-1-3-5-1

that adds to 13.

5-1-2-3-5-1

adds to 17.

So it's done that math for us,

we 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 500 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 prob, ugh,

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 that

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 gonna clear this out

we're done with sum of items

we don't need to worry about it

we're not gonna use the same expression

so when I 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 um

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 mean of items

and again, that very quickly,

let's me differentiate between

sum of items and mean of items,

type and label,

and 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 1

comma

2

comma

3

comma

4

comma

5

and then 6

I'm making sure to

not 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 items.

It's going to be the mean

of item 1, 2, 3, 4, 5, and 6

I'm going to click OK

SPSS again showed me

kind of the syntax 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 um 1-1-3-3-1-2

and a mean of 1.83 would

just eyeballing it

kind of make sense.

If we look at this one, 3.67

2-3-5-4-5-3

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.

Um so that is very quickly how to compute

a new variable in SPSS

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.