Week 8 Lecture 8 - correlation and partial correlation

Question 1

What does bivariate mean?

Answer 1

2 variables

Question 2

What do relationships vary in terms of?

Answer 2

• form
• direction
• magnitude/strength

Question 3

What are the assumptions for a linear correlation?

Answer 3

• both variables should be continuous (level of measurement)
• related pairs: each ppts should have a pair of values
• absence of outliers
• linearity –> points in the scatterplot should be best explained with a straight line

Question 4

If there are outliers on your scatterplot, what may it be appropriate to do?

Answer 4

remove the outlier
as it may not be representative of the larger population that you are sampling from

Question 5

True or false
correlations are sensitive to range restrictions

Answer 5

true
e.g., floor and ceiling effects

Question 6

If the assumptions for a linear correlation are seriously violated, what non-parametric equivalent should you use instead?

Answer 6

Spearman’s rho (or Kendall’s Tau if fewer than 20 cases)

Question 7

What letter is denoted to Pearson’s correlation coefficient?

Answer 7

r

Question 8

What does covariance provide?

Answer 8

• a measure of the variance shared between x an y variables
• r = ratio of covariance to separate variances
• we can obtain a measure of separate variances by multiplying the SD for x and y

Question 9

true or false
r is a ratio

Answer 9

true

Question 10

If covariance is large relative to the separate variances, what will r be?

Answer 10

further from 0

Question 11

If covariance is small relative to the separate variances, what will r be?

Answer 11

close to 0

Question 12

How does r reflect how well a straight line fits the datapoints?

Answer 12

if data points cluster close to the line, r will be further from 0
if data points cluster far from the line, r will be close to 0

Question 13

Does SPSS, give dof output?

Answer 13

no, you have to work it out
dof = N-2

Question 14

Do we get sampling error when considering correlations?

Answer 14

yes
we would get different r values for different samples within the same population

Question 15

What are some features of the r distribution?

Answer 15

• mean = 0
• sample deviates from 0 expressed in standard error units
• can determine probability of obtaining r value of a given magnitude when null is assumed to be true

Question 16

Does SPSS provide the 95% CIs on its output?

Answer 16

• no
• can obtain using a online calculator

Question 17

How many d.ps do you report r to?

Answer 17

3

Question 18

How many d.ps do you report CIs to?

Answer 18

2

Question 19

What does r^2 express?

Answer 19

the proportion of the separate variances that is shared
the remaining variance is split equally between x and y

Question 20

When asked about relative strength, what measure should you use?

Answer 20

r^2

Question 21

What is r^2 a measure of?

Answer 21

effect size

Question 22

What is a partial correlation?

Answer 22

relationship between 2 variables, whilst removing influence of a third given variable

Question 23

In partial correlation, if the 3rd variable decreases the r value and it is no longer significant, how would we interpret this?

Answer 23

3rd variable may explain relationship

Question 24

In partial correlation, if the 3rd variable decreases the r value but is still significant, how would we interpret this?

Answer 24

3rd variable may partially explain relationship

Question 25

In partial correlation, if the 3rd variable doesn’t decrease the r value, how would we interpret this?

Answer 25

3rd variable doesn’t influence the relationship