Certain assumptions must be met for the MANCOVA to be used appropriately:
1. Normality: For each group, each dependent variable must represent a normal distribution of scores. Furthermore, any linear combination of dependent variables must be normally distributed. Transformation or removal of outliers can help ensure this assumption is met.[2] Violation of this assumption may lead to an increase in Type I error rates.[3]
2. Independence of observations: Each observation must be independent of all other observations; this assumption can be met by employing random sampling techniques. Violation of this assumption may lead to an increase in Type I errorrates.[3]
3. Homogeneity of variances: Each dependent variable must demonstrate similar levels of variance across each independent variable. Violation of this assumption can be conceptualised as a correlation existing between the variances and the means of dependent variables. This violation is often called 'homoscedasticity'[4] and can be tested for usingLevene's test.[5]
4. Homogeneity of covariances: The intercorrelation matrix between dependent variables must be equal across all levels of the independent variable. Violation of this assumption may lead to an increase in Type I error rates as well as decreasedstatistical power.[3]