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Applied Statistics Using SPSS, STATISTICA, MATLAB and R

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B.2.9 F Distribution<br />

B.2 Continuous Distributions 451<br />

Description: The F distribution was introduced by Ronald A. Fisher (1890-1962),<br />

in order to study the ratio of variances, <strong>and</strong> is named after him. The ratio of two<br />

independent Gamma-distributed r<strong>and</strong>om variables, each divided by its mean, also<br />

follows the F distribution.<br />

Sample space: ℜ + .<br />

Density function:<br />

df1<br />

/ 2<br />

⎛ df1<br />

+ df 2 ⎞⎛<br />

df1<br />

⎞<br />

Γ⎜<br />

⎟<br />

2 ⎜<br />

df ⎟<br />

⎝ ⎠⎝<br />

2 ⎠<br />

f ( x)<br />

= df1,<br />

df2<br />

Γ(<br />

df1<br />

/ 2)<br />

Γ(<br />

df 2 / 2)<br />

( df1−2)<br />

/ 2<br />

x<br />

, x ≥ 0,<br />

( df1+<br />

df2<br />

) / 2<br />

⎛ df1<br />

2 ⎞<br />

⎜<br />

⎜1+<br />

x<br />

df ⎟<br />

⎝ 2 ⎠<br />

with df1, df 2 degrees of freedom.<br />

Distribution function:<br />

B. 30<br />

x<br />

= f<br />

0 df1,<br />

df2<br />

F ( x)<br />

df1, df2<br />

∫ ( t)<br />

dt . B. 31<br />

Mean:<br />

df 2<br />

µ = , df2 > 2.<br />

df 2 − 2<br />

Variance: σ 2 2<br />

2df<br />

2 ( df1<br />

+ df 2 − 2)<br />

=<br />

, for df2 > 4.<br />

2<br />

df1<br />

( df 2 − 2)<br />

( df 2 − 4)<br />

Properties:<br />

X 1 /( a1<br />

p1)<br />

1. X 1 ~ γ a1,<br />

p , X<br />

1 2 ~ γ a2<br />

, p ⇒<br />

~ f<br />

2<br />

2a1,<br />

2a<br />

.<br />

2<br />

X 2 /( a2<br />

p2<br />

)<br />

X / a X / µ<br />

2. X ~ β a,<br />

b ⇒ =<br />

~ f 2a,<br />

2b<br />

.<br />

( 1−<br />

X ) / b ( 1−<br />

X ) /( 1−<br />

µ )<br />

3.<br />

4.<br />

X ~ f a,<br />

b ⇒ 1/<br />

X ~ f b,<br />

a , as can be derived from the properties of the<br />

beta distribution.<br />

2<br />

2<br />

X /n1<br />

X ~ χ n , Y ~ χ , independent<br />

~<br />

1<br />

n X , Y<br />

⇒ f<br />

2<br />

n1,<br />

n .<br />

2<br />

Y /n2<br />

5. Let X1,…, Xn <strong>and</strong> Y1,…, Ym be n + m independent r<strong>and</strong>om variables such<br />

that X i ~ nµ<br />

1,<br />

σ <strong>and</strong> Y<br />

1 i ~ nµ<br />

2,<br />

σ .<br />

2<br />

n<br />

2 2<br />

( X − µ ) /( σ ) / ( m<br />

(<br />

2 2<br />

− µ ) /( mσ<br />

) ) ~ n m .<br />

∑ i= i n<br />

1<br />

Then ( 1 1 )<br />

∑ Y<br />

i i f<br />

= 1 2 2<br />

,<br />

6. Let X1,…, Xn <strong>and</strong> Y1,…, Ym be n + m independent r<strong>and</strong>om variables such<br />

that X i ~ nµ<br />

1,<br />

σ <strong>and</strong> Y<br />

1 i ~ nµ<br />

2,<br />

σ .<br />

2<br />

n<br />

2<br />

2 m<br />

2<br />

2<br />

Then ( ∑ ( X − − )<br />

i= i x)<br />

/(( n 1)<br />

σ 1<br />

1 ) / ( ∑ ( Y − − )<br />

i= i y)<br />

/(( m 1)<br />

σ 1<br />

2 ) ~ fn−1,m−1,<br />

where x <strong>and</strong> y are sample means.

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