Week Eleven - ANCOVA

Question 1

What is an ANCOVA?

Answer 1

A statistical procedure

Question 2

Normally, what are secondary variables controlled by?

Answer 2

elimination, constancy, or turning them into an IV.

Question 3

What are we doing with ANCOVA?

Answer 3

We are statistically controlling for the offending secondary variable by removing the variance associated with it.

eg adjusting everybody’s scores on the DV to be what we would think if everyone had the same score on the covariate

Question 4

What is the major benefit for ANCOVA?

Answer 4

It allows a more sensitive test of treatment effects as we are removing the variance that is due to differences on the covariate which reduces the amount of unexplained variance.

Question 5

What does an ANCOVA produce?

Answer 5

main effect of treatment
main effect of covariate
interaction between the treatment and CV

Question 6

What does the main effect of treatment test in an ANCOVA?

Answer 6

Tests whether the means of the groups are sig difference after controlling for the effect of the CV on the DV

• are the intercepts for the regression lines for each group the same? - if not: there is a sig difference between the groups on the DV when the CV is 0

Question 7

What does the main effect of covariate test in an ANCOVA?

Answer 7

Tests whether the CV is sig related to the DV - if not this is good cause CV not going to have impact ov DV scores.

• within the treatment groups, is there a correlation between the scores on the DV and the CV.

Question 8

What does the interaction between treatment (IV levels) and the CV test in an ANCOVA?

Answer 8

Tests whether the relationship between the CV and DV is the same for all treatment groups

A sig interaction would tell us whether the slope of the DV-CV regression line is the same for each treatment - if sig its not good (size of differences between groups will change)

Question 9

What is an important assumption of ANCOVA?

Answer 9

Parallel relationships between the covariate and the DV for both groups

Question 10

What happens if the slopes/regression lines are not parallel (non homo) what happens?

Answer 10

The size of the difference between groups is not going to be the same at the levels of the covariate

Question 11

What does ANCOVA not include?

Answer 11

Does not include an interaction term as it assumes the slope for all groups are the same

Question 12

When doing the analysis, when do you know that an ANCOVA is not the right analysis to be doing?

Answer 12

When you don’t have a significant main effect for the covariate

Question 13

What does an ANCOHET do?

Answer 13

It explicitly tests whether the slopes of the regression line between DV and CV are homogenous

Question 14

Assumptions of ANCOVA and ANCOHET?

Answer 14

All normal ANOVA ones plus

• linear relationships between CV and DV
• normal distribution of scores
• HOV
• independence of residuals
• homo of regression: parallel slopes

Question 15

What is the difference between ANCOVA and ANCOHET in regard to homogeneity of regression?

Answer 15

ANCOVA assumes it

ACOHET tests it

Question 16

What do we want/what is ideal for these kinds of tests? How do we test this?

Answer 16

We want independence of the CV and IV

Test by doing a normal one-way ANOVA with CV as DV and then IV as normal - if sig then they are NOT independent (bad)

Question 17

What happens if we have a lack of independence between the IV and the CV (so if they are related) and we control for the effects of the CV on the DV?

Answer 17

We make a type II error

Question 18

If a subsequent ANCOVA is sig following step down, how can it be interpreted? What about if not sig?

Answer 18

The DV adding important information in addition to the most important DV

if not sig, doesn’t add