北大暑期课程《回归分析》(Linear-Regression-Analysis)讲义PKU5教学文稿

Class 5: ANOVA (Analysis of Variance) and F-tests

I. What is ANOVA

What is ANOVA? ANOVA is the short name for the Analysis of Variance. The essence

of ANOVA is to decompose the total variance of the dependent variable into two additive components, one for the structural part, and the other for the stochastic part, of a regression. Today we are going to examine the easiest case.

II. ANOVA: An Introduction

Let the model be

εβ+= X y .

Assuming x i is a column vector (of length p) of independent variable values for the i th'

observation,

i i i εβ+='x y .

Then b 'x i is the predicted value.

sum of squares total:

[]

∑-=2

Y y SST i []

∑-+-=2

'x b 'x y Y b i i i

[][]

[][]

∑∑∑-+-+-=Y -b 'x b 'x y 2Y b 'x b 'x y 2

2

i i i i i i

[][]

∑∑-+=2

2

Y b 'x e i i

because [][][]∑∑=-=--0Y b 'x e Y b 'x b 'x y i

i i i i .

This is always true by OLS. = SSE + SSR

Important: the total variance of the dependent variable is decomposed into two additive parts: SSE, which is due to errors, and SSR, which is due to regression. Geometric interpretation: [blackboard ]

Decomposition of Variance

If we treat X as a random variable, we can decompose total variance to the between-group portion and the within-group portion in any population:

()()()i i i x y εβV 'V V +=

Prove:

()()

i i i x y εβ+='V V

()

()()

i i i i x x εβεβ,'Cov 2V 'V ++=

()()i

i

x εβV 'V +=

(by the assumption that ()

0 ,'Cov =εβk x , for all possible k.)

The ANOVA table is to estimate the three quantities of equation (1) from the sample.

As the sample size gets larger and larger, the ANOVA table will approach the equation closer and closer.

In a sample, decomposition of estimated variance is not strictly true. We thus need to

separately decompose sums of squares and degrees of freedom. Is ANOVA a misnomer?

III. ANOVA in Matrix

I will try to give a simplied representation of ANOVA as follows:

[]

∑-=2

Y y SST i (

)

∑-+=i i y Y 2Y y 2

2

∑∑∑-+=i i y Y 2Y y 2

2

∑-+=2

22

Y n 2Y n y i (because ∑=Y n y i )

∑-=2

2

Y n y i

2

Y n y 'y -=

y J 'y n /1y 'y -= (in your textbook, monster look)

SSE = e'e

[]

∑-=2

Y b 'x SSR i

()()[

]

∑-+=Y b 'x 2Y b 'x 2

2

i i

()[]

()∑∑-+=b 'x Y 2Y n b 'x 2

2

i i