Week 2 Lecture 2 - t-tests

Question 1

How many IVs and levels should there be to use a t test?

Answer 1

1 IV, 2 levels

Question 2

What do t-tests estimate?

Answer 2

Whether pop. means under 2 levels of IV are different

Question 3

What are the 2 types of variance?

Answer 3

variance between IV levels (ppts in different group getting different scores)
variance within IV levels (ppts in same group getting different scores)

Question 4

What contributes to variance between IV levels for an independent t-test?

Answer 4

• manipulation of IV
• individual differences
• experimental error (random error and constant error)

Question 5

What contributes to variance within IV levels for an independent t-test?

Answer 5

• individual differences
• experimental error (random error)

Question 6

What is the main point of interest for contributing factors to variance?

Answer 6

manipulation of the IV

Question 7

How would you find the mean difference between 2 sample means from 2 pops.

Answer 7

Mean A - Mean B = Mean D (difference)

Question 8

What are standard error units?

Answer 8

can express the extent to which an individual sampled mean difference deviates from 0

Question 9

How to calculate ESE for differences in an independent t-test?

Answer 9

ESE D = sqrt((s^2A / nA) +(s^2B/ nB))

Question 10

Is t a ratio?

Answer 10

yes - reflects the difference between the sample means, expressed in SE units

Question 11

How to calculate a t-stat for an independent t-test for difference?

Answer 11

t = mean D / ESE D

Question 12

A small variance between IV levels relative to within IV levels leads to a t value that is

Answer 12

close to 0

Question 13

A large variance between IV levels relative to within IV levels leads to a t value that is

Answer 13

further from 0

Question 14

What are degrees of freedom?

Answer 14

the difference between no. of measurements made and the number of parameters estimated

i.e., sample size - no. of parameters (n-1)

Question 15

True or false
the larger the DOF, the more reliable

Answer 15

true

Question 16

What is the dof for an independent t test?

Answer 16

n total - 2

Question 17

When dof = infinity, what is the critical t value?

Answer 17

1.96

Question 18

what are the 4 assumptions for an independent t test?

Answer 18

• normality
• homogeneity of variance
• equivalent sample size
• independence of observations

Question 19

What is normality for an independent t test?

Answer 19

DV is normally distributed, under each level of the IV

Question 20

What is homogeneity of variance for an independent t test?

Answer 20

the variance in the DV, under each level of the IV, should be reasonably equivalent

Question 21

What does Levene’s test check?

Answer 21

checking the assumptions of homogeneity of variance
testing to see if there’s a difference in variances
no difference = homogeneity
difference = heterogeneity

Question 22

What is equivalent sample size for an independent t test?

Answer 22

sample size under each IV level should be roughly equal
particularly important fir small samples

Question 23

What is independence of observations for an independent t test?

Answer 23

scores under each IV level should be independent
particularly important for quasi-experimental

Question 24

If the assumptions for an independent t test are violated, what should you do?

Answer 24

use the none parametric alternative - Mann-Whitney U test

Question 25

What is a paired t test for?

Answer 25

for within subjects designs

Question 26

How do you calculate mean D for a paired t test?/

Answer 26

= sum (xiA - xiB) / n

Question 27

How do you calculate ESE D for a paired t test?

Answer 27

s D / sqrt n

Question 28

What contributes to variance between IV levels in paired t tests?

Answer 28

• manipulation of IV
• experimental error

Question 29

What contributes to variance within IV levels in paired t tests?

Answer 29

• experimental error

Question 30

What are the assumptions of a paired t test?

Answer 30

• normality
• sample size

Question 31

What is normality in a paired t test

Answer 31

distribution of difference scores between IV levels should be approx normal
assume to be normal if n>30

Question 32

If the assumptions for an paired t test are violated, what should you do?

Answer 32

use non-parametric alternative –> Wilcoxon t test

Question 33

Can you determine if a result is likely to be significant using 95% confidence interval plots?

Answer 33

yes for independent groups
no for repeated measures

Question 34

What is the dof for a paired t test?

Answer 34

n-1

Question 35

What is Cohen’s D?

Answer 35

the magnitude of difference between 2 IV level means, expressed in SD units

i.e., a standardised value expressing the difference between the IV level means

Question 36

How do you calculate Cohen’s D?

Answer 36

(mean A - Mean B) / ((s A + s B) /2)

Question 37

how do you quantify effect sizes in Cohen’s D?

Answer 37

small = 0.2
medium = 0.5
large = 0.8

Question 38

What kind of overlap is there for a small and large effect size in Cohen’s D?

Answer 38

small = large overlap
large = small overlap