Week 10 (t-tests)

Question 1

When do we use a t test?

Answer 1

When we want to compare up to two sample means

Question 2

What are the type of t tests? (3)

Answer 2

One-sample
Independent-samples
Paired-samples

Question 3

What are the general assumptions of t-tests?

Answer 3

Data are interval or ratio DV

Data is normally distributed

Scores for an independent-samples test are independent

Equal variances (for independent samples test)

Question 4

What is a one-sample t-test?

Answer 4

Determines whether the sample mean is statistically different from a known or hypothesised population mean

Question 5

What occurs in a one-sample t-test?

Answer 5

The test variable is compared against a test value which is a known or hypothesised population mean

Question 6

How do we calculate the t-value for a one-sample t-test?

Answer 6

t = (mean from sample - expected mean)/estimated standard error

Question 7

When do we use an independent-samples t-test

Answer 7

Used when we can’t to compare means from two groups

Question 8

How do we calculate a t-value from and independent samples t test?

Answer 8

t = (X1-X2)/standard error of difference between means

X=
Sample means

Question 9

Does sample size affect t distribution?

Answer 9

Yes it varies with sample size

Question 10

How do we summarise an independent t-test?

Answer 10

t(n+n-2)= X, p= X, 95% CI [X,Y], d = x

Question 11

How do we find the confidence intervals for an independent t test?

Answer 11

Jamovi doesn’t calculate so use excel effect worksheet

Question 12

Explain confidence intervals overlap in independent t-tests?

Answer 12

If CI’s do not overlap more that 25% of their total length, the difference is statistically significant

Question 13

What is an expectation for paired-samples t-test?

Answer 13

Scores are independent

Pairs of scores will be correlated (however not a requirement)

Question 14

What is the null hypothesis for a paired-samples t test?

Answer 14

The mean of difference scores = 0

Question 15

What calculations occur in a paired-samples t-test?

Answer 15

There are two scores for each participant, one from each condition

Calculate difference scores for all participants

Calculate the mean difference score

Question 16

How do we calculate cohens d for a paired samples t-test?

Answer 16

Doesn’t take into consideration the correlation of two conditions so use effect size worksheet to calculate a better d

Question 17

What does the Shapiro-wilk test analyse?

Answer 17

Checks whether the data are normally distributed
If test is significant, then there is an issue with the normality of the data
If not significant, assume meets normality assumption (p values)

Question 18

How to calculate degrees of freedom for a paired samples t test?

Answer 18

N-1

Question 19

Should you report standard error in a paired samples t test?

Answer 19

Not normally reported so convert to SD by multiplying it by the square room of df+1

Question 20

Which t test do we use for a between subjects design?

Answer 20

An independent samples test

Question 21

Which t test do we use for a within subjects design?

Answer 21

A paired samples t test

Question 22

How do we test the assumption of homogeneity of variance in independent samples t tests?

Answer 22

Levene’s test

Question 23

When is the homogeneity of variance satisfied?

Answer 23

When the variances of the DV at the two levels of IV are equal or nearly equal

Question 24

What happens when levene’s test is not significant?

Answer 24

If the p value is not statistically significant it indicates the standard deviations for the two groups do not differ significantly

The data meets the assumption of normality

Question 25

If levene’s test is not significant which statistics do we report?

Answer 25

Students t stats

Question 26

What happens when levene’s test is significant?

Answer 26

If the p value is statistically significant it indicates the SD’s differ for the two groups and the assumption of homogeneity of variance has been violated

Question 27

Which statistics should be reported when the levene’s test is significant?

Answer 27

Stats from Welch’s t

Question 28

What is the difference between student’s t and Welch’s t?

Answer 28

Identical in terms of t statistic
Welch’s test compensates for the violation of homogeneity of variance by applying an adjustment to the degrees of freedom, making it smaller.

Question 29

Why does making the degrees of freedom smaller change the differences between the means?

Answer 29

The smaller the df for a t test, the larger the value of t test required for the difference between the means to be significant

Question 30

Which t-test require categorical IV’s?

Answer 30

Paired and independent t tests

Question 31

Welch’s test and errors?

Answer 31

More conservative, decreasing the risk of type 1 error but increases the risk of type 2 error.

Confidence intervals will be wider for this test