ANCOVA

• Assumptions

• Dependent variable and covariate variable(s) should be measured on a continuous scale
• Independent variable should consist of two or more categories
• Independence of observations
• No significant outliers
• Residuals should be approximately normally distributed for each category of the independent variable
• Homogeneity of variances
• Covariate should be linearly related to the dependent variable at each level of the independent variable.

• Hypothesis
  

Ho: main effect of independent variable_i is not significant.

ha: main effect of independent variable_i is significant.

• Critical value
  

F(1-a/2,n-1,n2-1), F(a/2,n1-1,n2-1)  

• P-value
  

2*(1-P(F≤|f|))  

• Decision rule
  

Reject Ho if F < F(1-a/2,n1-1,n2-1)  or F > F(a/2,n1-1,n2-1)