Lecture 4 - Mean Differences and Effect Size

Question 1

What are the 3 different types of differences between means?

Answer 1

One sample, related samples and independent samples

Question 2

What is a one sample difference?

Answer 2

The difference between the mean scores from one sample compared to a known value.

A known value could be for example; a national average, a population mean, or a neutral score.

Question 3

What is a related samples difference?

Answer 3

The difference between the means from two samples when both samples are from the same source. Eg. comparing scores from the same participants completing different tasks.

Question 4

What is an independent samples difference?

Answer 4

The difference between means from two independent samples. Eg. comparing scores from different groups of participants completing the same task.

Question 5

What are some other names for “related sample differences”?

Answer 5

Paired Samples / Matched Samples / Repeated Measures / Within Subjects

Question 6

What are some other names for “independent sample differences”?

Answer 6

Unpaired Samples / Unmatched Samples / Between Subjects

Question 7

Which formula is used to calculate the effect size for group differences between populations?

Answer 7

Cohen’s d

d = (μ₁ - μ₂) / σ

Key:
d = effect size
μ₁ = mean of population 1
μ₂ = mean of population 2
σ = population standard deviation

Question 8

Which formula is used to calculate the effect size for group differences between samples?

Answer 8

d = (M₁ - M₂) / s

Key:
d = effect size
M₁ = mean of sample 1
M₂ = mean of sample 2
s = sample standard deviation

Question 9

How do we know which standard deviation to use in an effects size formula?

Answer 9

If there is a population SD or a SD for a baseline/control group - use that.

If not, then add the SDs together for each sample and divide by 2 to find the average.*

• Formally this is a more complex calculation but for the purposes of our degree we just find the average.

Question 10

How would we describe an effect size of 0.2?

Answer 10

small

Question 11

How would we describe an effect size of 0.5?

Answer 11

medium

Question 12

How would we describe an effect size of 0.8?

Answer 12

large

Question 13

Which number approximately represents a small effect size?

Answer 13

0.2

Question 14

Which number approximately represents a medium effect size?

Answer 14

0.5

Question 15

Which number approximately represents a large effect size?

Answer 15

0.8