[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

____– three different kinds of
dependent variable can be used

Answer 5

Independent groups designs

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 Chi-Square 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: Normal Distribution and the
Independent Sample t-test

Answer 14

• 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 distribution of the two groups.

Question 15

Dispelling Myths
.

Answer 15

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 16

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

Answer 16

Box and whisker plot –

Question 17

We use the ____– approximately equal
sample sizes.

Answer 17

Pooled Variance t-test

Question 18

Use ____ instead of histograms.

Answer 18

box and whisker plot

Question 19

Check first the _____
(distribution of each group).

Answer 19

normal distribution assumption

Question 20

The general form of the t-tests follow a
____

Answer 20

similar pattern.

Question 21

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

Answer 21

difference
between two means; d

Question 22

The Equal Standard Deviations Assumption
* Also known as _____

Answer 22

Homogeneity of Variance.

Question 23

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

Answer 23

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

Question 24

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

Answer 24

pooled variance t-test.

Question 25

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

Answer 25

unpooled variance t-test.

Question 26

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

Answer 26

Levene’s test

Question 27

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 27

statistically significant

Question 28

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

Answer 28

non-significant result

Question 29

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

Answer 29

sample size.

Question 30

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

Answer 30

not very good at detecting differences

Question 31

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

Answer 31

small

Question 32

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

Answer 32

we do not really care.

Question 33

And when it is _____ when we do care.

Answer 33

not very good is precisely

Question 34

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

Answer 34

equal

Question 35

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

Answer 35

equal (or approximately equal)

Question 36

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

Answer 36

unequal

Question 37

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

Answer 37

Unpooled Variance t-test

Question 38

Sometimes known as ____which was developed
by ____

Answer 38

Welch’s Test ; Welch (1938)

Question 39

Reasons for having Unequal Sample Sizes

Answer 39

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 40

Effect Size for Independent Groups t-test

Answer 40

• We often describe the difference between two samples
  by stating the difference between the two means.
• However, often the scale that is used is not one that
  actually makes any sense (unless we are very familiar
  with the scale)
• How do we tell then if the difference is good or bad?

Question 41

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

Answer 41

difference between the two means.

Question 42

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

Answer 42

makes any sense

Question 43

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

Answer 43

Use a measure of Effect Size, and in the case of the
independent groups t-test, an appropriate measure of
effect size is called Cohen’s d.
* Cohen’s d = measure of how far apart the means of the
two samples are, in SD units.
* It does not matter what the range of possible scores is –
we are interpreting it in terms of SD.

Question 44

Use a measure of Effect Size, and in the case of the
independent groups t-test, an appropriate measure of
effect size is called __
_

Answer 44

Cohen’s d.

Question 45

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

Answer 45

Cohen’s d

Question 46

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

Answer 46

SD

Question 47

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

Answer 47

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

Question 48

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 48

0 (It can not be negative)

Question 49

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

Answer 49

The Mann-Whitney U Test

Question 50

The Mann-Whitney U Test

Answer 50

Compares two unrelated groups.

Question 51

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

Answer 51

means; difference in the means

Question 52

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

Answer 52

medians

Question 53

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

Answer 53

ϴ; theta