Are there covariates?

The purpose of this presentation is to help you determine if there are covariates in the problem you are working with.

The purpose of this presentation is to help you determine if there are covariates in the problem you are working with.

There ARE NO Covariates

or

There ARE Covariates

Here is an explanation of a covariate:

Let’s say you want to know the relationship between X and Y.

Let’s say you want to know the relationship between X and Y.

X YIs related to ?

Let’s say you want to know the relationship between X and Y.

X YIs related to ?

ZBut, you wonder if might explain this relationship in part.

So you control for Z

. . . in order to see what the relationship would be

between and

if you took out the effect of

. . . in order to see what the relationship would be

between and

if you took out the effect of

X Y

. . . in order to see what the relationship would be

between and

if you took out the effect of

X Y

Z

Let’s consider this example:

Let’s say you want to know if there is a relationship between socio-economic status and student ACT scores.

Let’s say you want to know if there is a relationship between socio-economic status and student ACT scores.

Socio-economic

statusIs related to ?ACT

Scores

Let’s say you want to know if there is a relationship between socio-economic status and student ACT scores.

Socio-economic

statusIs related to ?ACT

Scores

Parent statusBut, you wonder if might explain this relationship in part.

By parent status, we are referring to whether there are two parents or a single parent in the home.

Here is what the data set might look like:

Here is what the data set might look like:

Student Socio-economic Status1 = poor2 = middle class3 = wealthy

ACT Scores

A 3 26

B 2 22

C 3 25

D 1 21

E 1 19

F 2 21

G 1 25

Here is what the data set might look like:

Student Socio-economic Status1 = poor2 = middle class3 = wealthy

ACT Scores

A 3 26

B 2 22

C 3 25

D 1 21

E 1 19

F 2 21

G 1 25

Let’s say that the relationship between these two variables is .85

(which we will explain in another presentation - shows a strong

relationship)

Here is what the data set might look like:

Student Socio-economic Status1 = poor2 = middle class3 = wealthy

ACT Scores

A 3 26

B 2 22

C 3 25

D 1 21

E 1 19

F 2 21

G 1 25

But, what if parent status

plays an important role

in this relationship?

Here is what the data set might look like:

Student Socio-economic Status1 = poor2 = middle class3 = wealthy

ACT Scores

Parent Status1 = single2 = two

A 3 26 2

B 2 22 2

C 3 25 2

D 1 21 1

E 1 19 1

F 2 21 1

G 1 25 2

Here is what the data set might look like:

Student Socio-economic Status1 = poor2 = middle class3 = wealthy

ACT Scores

Parent Status1 = single2 = two

A 3 26 2

B 2 22 2

C 3 25 2

D 1 21 1

E 1 19 1

F 2 21 1

G 1 25 2

After controlling for Parent Status we find that

the correlation drops from .85

to .62

Here is what the data set might look like:

Student Socio-economic Status1 = poor2 = middle class3 = wealthy

ACT Scores

Parent Status1 = single2 = two

A 3 26 2

B 2 22 2

C 3 25 2

D 1 21 1

E 1 19 1

F 2 21 1

G 1 25 2This means that parent status plays an important role in explaining the relationship between Socio-

economic Status and ACT Scores

Here is what the data set might look like:

Student Socio-economic Status1 = poor2 = middle class3 = wealthy

ACT Scores

Parent Status1 = single2 = two

A 3 26 2

B 2 22 2

C 3 25 2

D 1 21 1

E 1 19 1

F 2 21 1

G 1 25 2

By controlling for Parent Status we are able to see the unique relationship between S.E.S and ACT Scores

when Parent Status is no longer a factor.

Here is what the data set might look like:

Student Socio-economic Status1 = poor2 = middle class3 = wealthy

ACT Scores

Parent Status1 = single2 = two

A 3 26 2

B 2 22 2

C 3 25 2

D 1 21 1

E 1 19 1

F 2 21 1

G 1 25 2

Parent Status is an example of what we call a “COVARIATE”.

Here are some of the expressions you will find in your word problem if a covariate is present:

Here are some of the expressions you will find in your word problem if a covariate is present:

CONTROL FOR

Here are some of the expressions you will find in your word problem if a covariate is present:

CONTROL FOR

HOLD CONSTANT

Here are some of the expressions you will find in your word problem if a covariate is present:

CONTROL FOR

HOLD CONSTANTELIMINATE THE

EFFECT OF

Here are some of the expressions you will find in your word problem if a covariate is present:

CONTROL FOR

HOLD CONSTANTELIMINATE THE

EFFECT OF

PARTIAL OUT

Here are some of the expressions you will find in your word problem if a covariate is present:

CONTROL FOR

HOLD CONSTANTELIMINATE THE

EFFECT OF

PARTIAL OUT

ADJUST FOR

Now examine your word problem and determine if . . .

Now examine your word problem and determine if . . .

There ARE NO Covariates

or

There ARE Covariates