Analyzing Data from Independent Groups: Continuous and Ordinal Measures Flashcards
Most common experimental design in psychological research.
Independent group designs
Independent group designs
Used for comparing ______ groups which are independent of one another – meaning there are different people in each group.
two or more
Can be used for _____ and ________ designs.
true and quasi-experimental
people are not assigned to conditions, as they already belong to different groups.
E.g., Males and Females, Normal and Abnormal etc.
Quasi-experimental designs
Independent groups designs – three different kinds of ______ can be used.
dependent variable
Avoids the problems inherent in repeated measures designs;
1.
2.
3.
practice effects
sensitization
carry-over effects
Disadvantage – more people, difficulty of matching controlled group with ______
experimental group
Statistical Tests for Independent Groups Data
- the independent groups t-test
- the mann-whitney test
- the chi-square test
also called the wilcoxon mann whitney test
the mann-whitney test
Use of test depends on data being tested.
the chi-square test
also known as the between subjects t-test or the two samples t-test.
the chi-test
Two (or Three) Assumptions about the Data
- Continuous/interval scale
- Data within each group is normally distributed. We need to make sure that the data are approximately normally distributed within each group.
- The SD of the two groups are equal.
there are actually two kinds of t-test, one makes the assumption of ,__________ and one does not.
assumption of homogeneity of variance
The one which makes the assumption is _____
easier.
Sample size must be above some value, such as 6, for the t-test to be valid
dispelling myths
Sample sizes must be balanced, or similar.
dispelling myths
To look at the overall distribution of data to determine whether the data are appropriate for an_______
independent samples t-test
distribution of two (or more) groups can be shown for the same amount of space.
Box and whisker plot
in calculation of the independent groups t-test use _______ instead of histogram
box and whisker plot
We use the ____ – approximately
equal sample sizes.
Pooled Variance t-test
step ____
Calculate the SD of each group.
step 1
step ___
Calculate the SD of the Difference.
step 2
Where na means the number of people in group A, and
nb is the number of people in group B.
formula for the SD of difference
step ___
Calculate the Standard Error of Difference
step 3
Step ____
Calculate the Confidence Intervals
step 4
in IG t-test, instead of being interested in
mean scores, we are now interested in the ____
between two means. So we call this d
difference
The Equal Standard Deviations Assumption also known as ____
Homogeneity of Variance.
Also known as Homogeneity of Variance.
The Equal Standard Deviations Assumption
Two versions of the t-test:
Pooled Variance t-test
Unpooled Variance t-test
Two versions of the t-test: the _____,
which makes this assumption,
Pooled Variance t-test
Two versions of the t-test: the _____, which does not make this assumption.
Unpooled Variance t-test
If you have (approximately) equal sample sizes in your
two groups, use the ____
pooled variance t-test.
If you don not have (approximately) equal sample sizes
in the two groups, use the _____
unpooled variance t-test.
used to decide if variances (or SDs) are the same. (most common)
levene’s test
If Levene’s test give gives a statistically significant result, this means variances are different from one another and the _____ should be used. (vice versa)
unpooled variance test
Problem however is that a ____ does not mean that the variances are the same.
non-significant result
And the problems with the tests, such as the Levene test, is that they are _____on the sample size.
dependent
When the sample size is small, it is not very good at detecting differences in the ____. (vice versa)
variances
It only matters however that the variances are the same when the sample size is _____
small.
So, when Levene’s test is good at telling us when the variances are different precisely we do not really ____
nd when it is not very good is precisely when we do_____
care.
_____ does not really matter when the sample sizes are about equal.
Homogeneity of variance
So, if we have equal (or approximately equal) sample sizes, we can ignore the assumption of Homogeneity of Variance and use the _____
Pooled Variances t-test.
A modification of the t-test, which does not make the assumption of equal variances.
- Sometimes known as Welch’s Test which was developed by Welch (1938)
unpooled variance test
unpooled variance test sometimes known as __ which was developed by welch (1938)
Welch’s Test
Reasons for having Unequal Sample Sizes
Comparison of two naturally occurring groups, and they are different sizes.
It may be difficult or expensive for one of the interventions.
There may be an ethical or recruitment issue.
We often describe the difference between two samples by stating the difference between the ____
two means.
However, often the ____ 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?
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
measure of how far apart the means of the two samples are, in SD units.
Cohen’s d
It does not matter what the range of possible scores is – we are interpreting it in terms of ____
SD
Cohen’s d is often interpreted according to the following Rules:
- Large effect size: d = 0.8
- Medium effect size: d = 0.5
- Small effect size: d = 0.3
It can be as low as 0 (It can not be negative) and, unlike a correlation (which can not be higher than 1), it has no upper limit, although values above ___are rare.
1
Compares two unrelated groups.
- Used when independent samples t-test can not be used.
- Non-parametric test
The Mann-Whitney U Test
The t-test compares the means of two groups and tells us whether the difference in the means is _____
statistically significant.
The Mann-Whitney test does not compare means so it might be tempting to say that it compares _____Unfortunately it does not (necessarily
medians.
Use the value called ___ which is the Greek Letter theta
ϴ
effect size for the mann-whitney test
