EXPLAINING: EXPERIMENTS
Review:
1

Of the four goals of psychology, the one that means “cause &
effect” is to ____________ behavior.

“Explaining” requires that we have at least _____ (insert a
number) variables.

An __________ is the only study that can explain behavior.

This is because an experiment is the only study that is set up
so there is only ______ (insert a number) possible cause of the
effect.
(Answers: explain; two; experiment; one)
EXPLAINING: EXPERIMENTS
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
The study that is done to explain is called an
experiment.

However, it’s only legitimate to call a research study
an experiment if it follows several principles, rules
and procedures.

By the end of this show, you should be able to
describe and explain the rules, principles and
procedures.
EXPLAINING: EXPERIMENTS
An Introduction to the MAIN PRINCIPLE Underlying
an Experiment:

An experiment is the only type of study that
eliminates all variables, except one, that might
influence the effect.

In other words, the procedures in an experiment get
rid of all possible causes of an effect, except the one
we are trying to study.
* The complete version will be on slide 12.
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EXPLAINING: EXPERIMENTS
Three Rules that Help Us Eliminate Unwanted
Variables:

To make sure there is only one variable that might cause the
effect, the experimenter (or assistants) must be the one who
–
–
–
literally creates two (or more) identical groups of
participants,
determines which group of participants gets which amount
of the variable we think is causing the effect, and
creates & maintains an environment where there is only
one difference (the cause) between groups.
* Remember, cause & effect refers to the Goal called Explaining.
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EXPLAINING: EXPERIMENTS
A Sample Study – My Hypothesis
5

Typically, sports teams are more likely to win if they
play in their own city, their own stadium. It’s often
said this is because their fans are there to cheer
them on.

Let’s say I want to test the idea that “cheering fans”
are the cause of the home team advantage (the
effect).
EXPLAINING: EXPERIMENTS
A Sample Study – Following the Rules & Procedures
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
In my hypothetical experiment, I’m going to make
two sets of clones of your favorite football team.

For each of the two clones (per player) I’m going to
have them choose straws out of a hat. There will be
one short straw and one long straw for each pair.

If the person gets the short straw, he will be on Team
#1 and a long straw means Team #2.
EXPLAINING: EXPERIMENTS
A Sample Study – Continued


I will create two identical football stadiums, and
identical cloned fans. In Stadium 1, Team #1 will
play, and I will ensure that half the cloned fans will be
there and will cheer them on. In Stadium 2, Team #2
will play for the other half of the clones. This time,
though, I will not allow the fans to cheer.
I will then see which team wins. (I’m measuring the
“effect”.)
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EXPLAINING: EXPERIMENTS
NOTICE: I MET THE FIRST RULE: I must create two
identical groups.

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I personally split the clones into the two teams, and I
did it in such a way that they were exactly the same
as each other.
EXPLAINING: EXPERIMENTS
NOTICE: I MET THE SECOND RULE BY
FOLLOWING TWO PROCEDURES:
1. I personally created identical environments. Both teams had
the same crowds, (except for the cheering) and the stadiums
were the same.
2. I further kept out extra variables when I personally decided
which team would have a cheering crowd. When I choose, I
keep the teams from choosing. Later, we’ll discuss how this
eliminates unwanted possible causes.
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Vocabulary: If the experimenter actually creates the difference (in
the variable we think is causing something to happen), that
difference/variable is called the independent variable (i.v.).
EXPLAINING: EXPERIMENTS
REMINDER:
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
Only if these three procedures are followed should
we refer to the study as an experiment.

Why? Because only if these procedures are followed
do we rule out variables that might produce an effect.
EXPLAINING: EXPERIMENTS
Why Do We Need to Rule Out Extra Variables?
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
If there is more than one initial difference between
groups, we won’t know which difference produced
any effect we may later discover.

For example, if Stadium 1 had been colder than Stadium 2 and
Team #1 had won, we wouldn’t know if they won because of the
temperature or because of the cheering.
EXPLAINING: EXPERIMENTS
THE COMPLETE VERSON OF THE MAIN POINT:

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An experiment is the only type of study that
eliminates all variables (except the independent
variable) that might affect the outcome. Therefore, if
the two groups later differ on the “effect”, we know it
has to be because of the i.v. because there is no
other option. This is why an experiment is the only
type of study that can tell us about cause & effect.
Vocabulary: The “effect” is called the dependent variable (d.v.); its
amount is dependent on how much of the i.v. there was.
Creating Identical Groups
Definitions: Identical groups
1. Two (or more) groups that are the same as
each other with regards to the types of people
making up the groups.
2. Two (or more) groups of people where the
groups are identical to each other regarding any
possible personal variable we could think of.
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Hmmm….. How Do We Get Identical
Groups?



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We can’t really clone people.
Where are we going to find many, many sets
of twins that we can separate into 2 groups
so the two groups are identical to each
other?
CAN WE ONLY DO EXPERIMENTS ON TWINS?
Creating Identical Groups – The Difficult
Way
15

Well, no. I suppose instead we could get two
females and put one in each group. Then the
groups would have identical numbers of
females.

But, what if one female was shy and the
other wasn’t? Then the groups differ on
“shyness.”
Creating Identical Groups – The Difficult
Way
16

Okay, now our groups differ on shyness.
What can we do about that?

I guess we could get four females, two who
are shy and two who are not. We could put
one of each in each group. That is, in Group
#1, we’d have one shy & one not shy female,
and in Group #2, we’d have one shy & one
not shy female.
Creating Identical Groups – The Difficult
Way

But then, what if only one of them has kids?
The groups wouldn’t be equal anymore.
REVIEW:
Explain why we don’t want parents in one
group but not in the other. _____________
____________________________________.
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Creating Identical Groups – The Difficult
Way
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
But what if our first group has a female who
is Catholic and diabetic but our second group
does not?

YIKES!!! There must be an easier way to
create identical groups!!!
Creating Identical Groups
A MNEMONIC

Have you noticed the picture in the right hand corner
of the last few slides?

Can you think of another name for a donkey?
(If English isn’t your first language, you may need to ask
someone.)
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
Shhh…. Don’t say it out loud! Write it down the other
word: __________.

Yes! This IS related to identical groups!
(donkey)
Creating Identical Groups: Use Random
Assignment

To create identical groups, we will use a procedure
called random assignment.

(We will think about donkeys so we don’t confuse random
assignment with random selection. If you confuse them on
assignments or tests, you will lose points.)

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Here – let’s practice getting it right: To get identical groups,
we will use random _____ignment. 
(donkey)
Creating Identical Groups – Use Random
Assignment
Definition: RANDOM ASSIGNMENT
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–
After we know which people will be in the
experiment, we divide them into the two groups
using some method of chance (randomly). For
example, we could flip a coin. “Heads – you’re in
group A”. “Tails – you’re in group B.”
–
This procedure is called Random Assignment.
Creating Identical Groups: Use Random
Assignment

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VERY IMPORTANT POINT
If we want to know about cause & effect,
random assignment is a crucial procedure for
creating identical groups. This is because
random assignment creates two groups
that are identical on their personal
characteristics and histories (theoretically, at least).
How Does Random Assignment Create Identical
Groups?

Consider this: When we choose using “heads or tails”, we
would expect half the participants to get heads and half to get
tails. So, we are required to assume that
–
–

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half the females were assigned to one group and half were
assigned to the second group.
half the males were assigned to the first group and half to the
second group.
So, now the two groups are identical with regards to “sex.”
Then, if the two groups differ on the outcome (the d.v.), we
know it can’t be because of “sex”, since “sex” was made
constant across the two groups. (In other words, it didn’t vary.)
How Does Random Assignment Create Identical
Groups? continued

This means all personal qualities such as age,
shyness, hormone levels, town grown up in, number
of good teachers previously taken, right vs. left
handed, level of ADHD, GPA, frequency of drinking
water, amount of serotonin, type of car driven, age of
first kiss, level of shyness etc. etc. etc. cannot be
the cause of a change in the dependent variable.
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(Yes, I know….

You may be thinking that in real life, coins don’t always come
up heads half the time and tails half the time. This is true, and
after every experiment is finished, the results are analyzed with
sophisticated statistics that take this into account.

I will not be asking you to remember this. It’s just to satisfy your
curiosity.)
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Remember this really important main
principle?
If we have two identical groups who differ only on the
independent variable and if
 we find a difference when we measure the
dependent variable (d.v.),
 the only possible explanation for the d.v. being
different between groups is ____________________
___________________________. (You fill it in. If you
can’t, you’re missing the main point, so go back over
the material.)

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Creating Identical Groups: Use Random
Assignment
IMPORTANT POINTS:
 Remember, creating identical groups with random
assignment rules out unwanted “personal” variables
as potential causes of the dependent variable. If a
study does not use random ___ignment, we know
with certainty that it has not ruled out multitudes of
variables that could affect the d.v.

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Since identical groups, and thus no unwanted
variables, are essential if we are to know about
cause and effect, random ____ignment also is
essential.
Creating Identical Groups: Random Assignment

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Without random ____ignment, a study is
not a true experiment.
Identical Groups – Another Required Procedure
IMPORTANT POINT

29
After using random assignment, to keep the
groups identical they must be treated
identically, except with regards to the
independent variable.
Research & Goal #3 - Explaining
EXPERIMENTS
Identical Groups
REWORDING THE MAIN POINT
 Because a study should only be called an
experiment if groups are identical (other than
the i.v.), and because having identical groups
rules out other possible causes that might
affect the d.v., an experiment is the only type
of study that can explain.
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On a Different Note:
Random Selection
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
Random Selection can be used with any type of
study.

Random selection often is not used because it
can be difficult, or we may have little choice
regarding who will be in our study.

Random selection is irrelevant for testing
cause & effect.
Random Selection
32

Poor Ole’ Random Selection. It tells us
absolutely nothing about cause and effect.

What it is good for is giving us participants who
represent the group of people we are trying to
understand, called the population.
Random Selection - Definition
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
Random Selection is using some method of
chance to choose the people who will be
participants in the study.

IF Random Selection is used in an experiment, it
is done before random assignment.

This is because random assignment cannot be
done unless we already know who will be in our
experiment.
Random Selection

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Random Assignment
Group 1

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Group 2