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Lecture 6 statistiek 4.4
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Lecture 6 statistiek blok 4.46A: ANCOVA DEEL 2
Stappen ANCOVA:
1. Measurement level of the variables
2. Check for outliers
3. Check assumptions
• Normality (for each group)
• Homoscedasticity
• Linearity
• Homogeneity of regression slopes
• Independence of treatment and covariate
• (maybe) multicollinearity
4. Check quality of prediction
Checking for outliers: univariate à save à leverage,
cooks, standardized residuals.
Als je niet zeker bent welke je moet kiezen
dan voor het grotere criterium (3(k+1)/n.
Normally distributed errors à als groep varianties gelijk zijn, kunnen we de assumptie overall
kijken (niet per groep). SPSS: save standardized residuals, maak histogram (check voor outliers)
Homoscedasticity (errorvarianties in elke groep moeten gelijk zijn) à Levene’s test.
Als Levene’s test sign.
is dan significant
verschil in
errorvarianties. Dit wil
je niet!!
1
, Lecture 6 statistiek blok 4.4
Linearity (tussen covariate en dependent variabele) à bij elke groep! Graphs à scatterplot
X-as: covariate
Y-as: dependent variable
Er is een lineaire relatie voor de drie groepen.
Homogeneity of regression slopes (bij second continue variabele;covariate)
à Heel belangrijk, VIOLATED? à GEEN ANCOVA
1. See parallel regression lines in previous scatterplot
2. Test violation of assumption by: repeating ANCOVA en including interaction effect:
covariate x group
o Analyze à General Lineair Model à Univariate à model à build terms à
(group en T0QL tegelijk selecteren en dan met het pijltje naar links verschuiven
dan krijg je het interactie effect).
o Sign? dan interactie en kan je stoppen want dan zijn de lijnen niet paralell
o Xsign? dan kan je verder gaan dan is er geen interactie tussen group en covariaat,
dus geen indicatie voor verschillende regressieslopes.
Haal interactie-effect er weer uit, want beïnvloed F-value. Dit doe je door bij model op full-
factorial te klikken.
Independence of treatment and covariate (bij second continue variabele; covariate)
Average size of covariate moet hetzelfde zijn voor de verschillende groepen.
p > .05 dit wil je want dan
geen verschil.
(maybe) multicollinearity (bij second continue variabele;covariate)
= different predictor variabelen niet correleren. Multiple covariates Xcorrelated.
2
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