Question

In: Statistics and Probability

The following 12 data pairs relate variable xi, the amount of fertilizer, to variable Yi, the...

  1. The following 12 data pairs relate variable xi, the amount of fertilizer, to variable Yi, the amount of wheat harvested:

x: 30 30 30 50 50 50 70 70 70 90 90 90

Y: 9 11 14 12 14 23 19 22 31 29 33 35

such that : ∑x = 720 ∑y = 252 , ∑ xy =17240, ∑x2 =49200, ∑y2 = 6228

a) Find equation of linear regression line: Y = A + BX.

b) 95% 2 sided confidence interval for B.

c) Is there regression on input variable?

d) Find 2-sided 99% prediction interval for response if x0 = 40.

e) Calculate R2, explain its meaning.

Solutions

Expert Solution

a)

X Y XY
total sum 720 252 17240 49200 6228
mean 60.0000 21.0000

sample size ,   n =   12          
here, x̅ =Σx/n =   60.000   ,   ȳ = Σy/n =   21.000  
                  
SSxx =    Σx² - (Σx)²/n =   6000.00          
SSxy=   Σxy - (Σx*Σy)/n =   2120.00          
SSyy =    Σy²-(Σy)²/n =   936.00          
estimated slope , ß1 = SSxy/SSxx =   2120.000   /   6000.000   =   0.35333
                  
intercept,   ß0 = y̅-ß1* x̄ =   -0.20000          
                  
so, regression line is   Ŷ =   -0.200   +   0.353   *x

b)

confidence interval for slope                  
α=   0.05              
t critical value=   t α/2 =    2.228   [excel function: =t.inv.2t(α/2,df) ]      
estimated std error of slope = Se/√Sxx =    4.32358   /√   6000.00   =   0.056
                  
margin of error ,E= t*std error =    2.228   *   0.056   =   0.124
estimated slope , ß^ =    0.3533              
                  
                  
lower confidence limit = estimated slope - margin of error =   0.3533   -   0.124   =   0.229
upper confidence limit=estimated slope + margin of error =   0.3533   +   0.124   =   0.478

c) yes

d)

X Value=   40                      
Confidence Level=   99%                      
                          
                          
Sample Size , n=   12                      
Degrees of Freedom,df=n-2 =   10                      
critical t Value=tα/2 =   3.169   [excel function: =t.inv.2t(α/2,df) ]                  
                          
X̅ =    60.00                      
Σ(x-x̅)² =Sxx   6000.000000                      
Standard Error of the Estimate,Se=   4.32                      
                          
Predicted Y at X=   40   is                  
Ŷ =   -0.200   +   0.353   *   40   =   13.933
                          

For Individual Response Y                          
standard error, S(ŷ)=Se*√(1+1/n+(X-X̅)²/Sxx) =   4.6365                      
margin of error,E=t*std error=t*S(ŷ)=    3.1693   *   4.64   =   14.6944      
                          
Prediction Interval Lower Limit=Ŷ -E =   13.933   -   14.694   =   -0.7611      
Prediction Interval Upper Limit=Ŷ +E =   13.933   +   14.694   =   28.6277      

e)

R² =    (Sxy)²/(Sx.Sy) =    0.8003

about   82.16%   of variation in observation of variable Y, is explained by variable x


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