Analysis of covariance

When…

ANCOVA is an ‘extra’ on an ANOVA

ANOVAoutcome = number of words learnedIV = Sex

ANCOVA adds a covariatecovariate = size of first language vocabulary

Control

Variables that influence the outcome

Manipulate

Controlhold steadyrandomiseenter into the statistical model

Control

Hold steadyEveryone has same L1 vocabMean of groups is same for L1 vocab

Randomisesometimes you can’t (e.g. Sex)

ANCOVA – record L1 vocab and allow statistically

ANCOVA in SPSS

Use GLM / Univariate

Enter outcome as dependent variable

Enter Sex as ‘Fixed’ IV

Enter L1 vocab as ‘covariate’

ANCOVA assumptions

Linearityrelation between covariate and outcome is

(roughly) linear

‘Homogeneity of regression ’relation between covariate and outcome is the

same for both Men and Women [relation is the same at each level of the IV]

Checking Homogeneity of regression in SPSS

A second ‘run’ through Analyse/GLM/univariate

Click the ‘model’ button’

Select IV (Sex) and Covariate and ‘click across’;Now select both together and ‘click across’;Continue as before….

Interaction effect – must not be significant

MANOVA

Multiple dependent variables‘converted’ into one composite DVAssumptions – see Pallant SPSS Survival Guide(similar to Multiple Regression: Linearity, Mahalanobis distances) Plus homescedasticity (use Box’s M with a conservative p-value)

Further reading: Tabachnik & Fidell