[L6] Analyzing Data from Independent Groups: Continuous and Ordinal Measures

Question 1

• Most common experimental design in psychological
  research.

Answer 1

Independent Group Designs

Question 2

Independent Group Designs- Used for comparing two (or more) groups which are
independent of one another – meaning there are ___
__
_ in each group.

Answer 2

different people

Question 3

Independent Group Designs - Can be used for _____

Answer 3

true and quasi-experimental designs.

Question 4

____ people are not assigned to
conditions, as they already belong to different groups.

Answer 4

Quasi-experimental designs –

Question 5

Independent groups designs– __ different kinds of
__ can be used

Answer 5

three; dependent variable

Question 6

Advantages of Independent Group Designs

Answer 6

Avoids the problems inherent in repeated measures
designs; practice effects, sensitization or carry-over
effects.

Question 7

Disadvantage Independent Group Designs

Answer 7

more people, difficulty of matching
controlled group with experimental group

Question 8

Statistical Tests for Independent Groups
Data

Answer 8

1. The Independent Groups t-test
2. The Mann-Whitney Test
3. The Chi-Square Test

Question 9

Use of test depends on data being tested.

Answer 9

Statistical Tests for Independent Groups
Data

Question 10

The Mann-Whitney Test (also called the

Answer 10

Wilcoxon-
Mann Whitney Test)

Question 11

also known as the between subjects t-test or the twosamples
t-test.

Answer 11

The Independent Groups t-test

Question 12

Two (or Three) Assumptions about the Data

Answer 12

1. Continuous/interval scale
2. Data within each group is normally distributed. We
   need to make sure that the data are approximately
   normally distributed within each group.
3. The SD of the two groups are equal

Question 13

there are actually two kinds of t-test, one makes
the assumption of ___, and one
does not.

Answer 13

homogeneity of variance

Question 14

Common Mistake:

Answer 14

Normal Distribution and the
Independent Sample t-test

Question 15

• To look at the overall distribution of data to determine
  whether the data are appropriate for an independent
  samples t-test.
• It should be the ___ of the two groups.

Answer 15

distribution

Question 16

Dispelling Myths
.

Answer 16

1. Sample size must be above some value, such as 6, for
   the t-test to be valid.
2. Sample sizes must be balanced, or similar

Question 17

distribution of two (or more)
groups can be shown for the same amount of space.

Answer 17

Box and whisker plot –

Question 18

We use the ____– approximately equal
sample sizes.

Answer 18

Pooled Variance t-test

Question 19

Use ____ instead of histograms.

Answer 19

box and whisker plot

Question 20

Check first the _____
(distribution of each group).

Answer 20

normal distribution assumption

Question 21

The general form of the t-tests follow a
____

Answer 21

similar pattern.

Question 22

Difference – in IG t-test, instead of being interested in
mean scores, we are now interested in the ____. So we call this ___

Answer 22

difference
between two means; d

Question 23

The Equal Standard Deviations Assumption
* Also known as _____

Answer 23

Homogeneity of Variance.

Question 24

Two versions of the t-test: the ____
which makes this assumption, and the _____ which does not make this assumption.

Answer 24

Pooled Variance t-test, Unpooled
Variance t-test,

Question 25

If you have (approximately) equal sample sizes in your
two groups, use the ____

Answer 25

pooled variance t-test.

Question 26

If you don not have (approximately) equal sample sizes
in the two groups, use the ____

Answer 26

unpooled variance t-test.

Question 27

___ used to decide if variances (or SDs) are
the same. (most common)

Answer 27

Levene’s test

Question 28

If Levene’s test give gives a ____
result, this means variances are different from one
another and the unpooled variance test should be used.
(vice versa)

Answer 28

statistically significant

Question 29

Problem however is that a ____does not
mean that the variances are the same.

Answer 29

non-significant result

Question 30

And the problems with the tests, such as the Levene test,
is that they are dependent on the _____

Answer 30

sample size.

Question 31

When the sample size is small, it is _____ in the variances. (vice versa)

Answer 31

not very good at detecting differences

Question 32

It only matters however that the variances are the same
when the sample size is ____.

Answer 32

small

Question 33

So, when Levene’s test is good at telling us when the
variances are different precisely ____

Answer 33

we do not really care.

Question 34

And when it is _____ when we do care.

Answer 34

not very good is precisely

Question 35

Homogeneity of variance does not really matter when
the sample sizes are about ____

Answer 35

equal

Question 36

So, if we have _____sample
sizes, we can ignore the assumption of Homogeneity of
Variance and use the Pooled Variances t-test.

Answer 36

equal (or approximately equal)

Question 37

When the sample sizes are ___, Homogeneity of
Variance matters a lot more.

Answer 37

unequal

Question 38

• A modification of the t-test, which does not make the
  assumption of equal variances.

Answer 38

Unpooled Variance t-test

Question 39

Sometimes known as ____which was developed
by ____

Answer 39

Welch’s Test ; Welch (1938)

Question 40

Reasons for having Unequal Sample Sizes

Answer 40

1. Comparison of two naturally occurring groups, and
   they are different sizes.
2. It may be difficult or expensive for one of the
   interventions.
3. There may be an ethical or recruitment issue.

Question 41

• However, often the_
  __ that is used is not one that
  actually makes any sense (unless we are very familiar
  with the scale)

Answer 41

scale

Question 42

We often describe the difference between two samples
by stating the
___

Answer 42

difference between the two means.

Question 43

However, often the scale that is used is not one that
actually
____ (unless we are very familiar
with the scale)

Answer 43

makes any sense

Question 44

How do we tell then if the difference is good or bad?

Answer 44

Use a measure of Effect Size,

Question 45

in the case of the
independent groups t-test, an appropriate measure of
effect size is called __
_

Answer 45

Cohen’s d.

Question 46

—= measure of how far apart the means of the
two samples are, in SD units.

Answer 46

Cohen’s d

Question 47

It does not matter what the range of possible scores is –
we are interpreting it in terms of _
_.

Answer 47

SD

Question 48

Cohen’s d is often interpreted according to the following
Rules:

Answer 48

• Large effect size: d = 0.8
• Medium effect size: d = 0.5
• Small effect size: d = 0.3

Question 49

It can be as low as ____ and, unlike
a correlation (which can not be higher than 1), it has no
upper limit, although values above 1 are rare.

Answer 49

0 (It can not be negative)

Question 50

*
* Used when independent samples t-test can not be used.
* Non-parametric test

Answer 50

The Mann-Whitney U Test

Question 51

The Mann-Whitney U Test

Answer 51

Compares two unrelated groups.

Question 52

The t-test compares the ___ of two groups and tells us
whether the
___ is statistically
significant.

Answer 52

means; difference in the means

Question 53

The Mann-Whitney test does not compare means so it
might be tempting to say that it compares _
__.
Unfortunately it does not (necessarily)

Answer 53

medians

Question 54

Effect Size for the Mann-Whitney test
* Use the value called ___, which is the Greek Letter ___.

Answer 54

ϴ; theta