Question

The manager of a fleet of automobiles is testing two brands of radial tires and assigns one tire of each brand at random to the two rear wheels of eight cars and runs the cars until the tires wear out. The data (in kilometers) follow. Find a 99% confidence interval on the difference in the mean life.
Car Brand 1 Brand 2
1 36663 33866
2 43509 41829
3 36240 35500
4 32100 31950
5 37210 38015
6 48360 47800
7 38200 37810
8 33500 33215
a) Calculate SD =
b) Calculate a 99% two-sided confidence interval on the difference in mean life.
c) Which brand would you prefer? (brand 1/ no difference /brand 2)_____

Answer 1

Answer:

a) σ  =  4933,64

b) CI 99%  = ( - 5746  ;  7194 )

c) No difference in brands

Step-by-step explanation:

Brand 1:

n₁   =  8

x₁   = 38222

s₁   = 4974

Brand 2:

n₂  = 8

x₂  = 37498

s₂  = 4893

As n₁  =  n₂  = 8       Small sample  we work with t -student table

degree of freedom      df  = n₁  +  n₂  - 2    df = 8 +8 -2  df = 14

CI = 99 %   CI  =  0,99

From  t-student table we find   t(c)  = 2,624

CI  =   (  x₁  -  x₂ ) ±  t(c) * √σ²/n₁   +  σ²/n₂

σ² = [( n₁  -  1 ) *s₁² +  ( n₂  -  1  ) * s₂² ] / n₁ +n₂ -2

σ² = 7* (4974)² + 7*( 4893)² / 14

σ² = 24340783       σ  =  4933,64

√ σ²/n₁  +  σ²/n₂     =  √ 24340783/8   +  24340783/8

√ σ²/n₁  +  σ²/n₂     =  2466

CI 99%  =  (  x₁  -  x₂ ) ± 2,624* 2466

CI 99%  =   724  ± 6470

CI 99%  = ( - 5746  ;  7194 )

As we can see CI 99% contains 0 and that means that there is not statistical difference between mean life of the two groups