From: Ted Harding <Ted.Harding_at_manchester.ac.uk>

Date: Sun, 02 Mar 2008 21:05:54 +0000 (GMT)

*> [2,] 0.1083491
*

*>
*

*> When I do it in Matlab I get theta to be
*

*> 79.69
*

*> 0.18
*

*>
*

*> which is correct. ~79 is the crossing of the y-axis.
*

*>
*

*> Have I perhaps written theta wrong? The formula for theta is
*

*> (alpha,beta)^T = (x^T * x)^(-1) * x^T * Y
*

*>
*

*> where ^T means transposed.
*

# [1,] 0.50 0.25

# [2,] 0.25 0.20

# [1,] 1 1.110223e-16

# [2,] 0 1.000000e+00

(Note the slight rounding error); whereas

E-Mail: (Ted Harding) <Ted.Harding_at_manchester.ac.uk> Fax-to-email: +44 (0)870 094 0861

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Sun 02 Mar 2008 - 21:13:57 GMT

Date: Sun, 02 Mar 2008 21:05:54 +0000 (GMT)

On 02-Mar-08 20:18:29, Louise Hoffman wrote:

> Dear readers

*>
**> I would like to make General Linear Model (GLM) for the
**> following data set http://louise.hoffman.googlepages.com/fuel.csv
**>
**> The code I have written is
**>
**> fuelData<-read.table('fuel.csv',header=TRUE, sep=',')
**> n<-dim(fuelData)[1]
**> xOnes<- matrix(1,nrow=n,ncol=1)
**> x<-cbind(xOnes,fuelData[,3])
**> y<-fuelData[,4]
**> theta<-((t(x)%*%x)^(-1))%*%t(x)%*%y
**>
**> which gives
**>
*

>> theta> [1,] 215.8374077

> [,1]

Unfortunately, x^(-1) is not the inverse of x:

x<-matrix(c(2,4,4,5),nrow=2)

x

# [,1] [,2] # [1,] 2 4 # [2,] 4 5 x^(-1) # [,1] [,2]

# [1,] 0.50 0.25

# [2,] 0.25 0.20

i.e. it is the matrix which you get by applying the operation (...)^(-1) to each element of x.

In R, the inverse of a non-singular matrix is (somewhat obscurely) denoted by solve(x):

solve(x)

# [,1] [,2] # [1,] -0.8333333 0.6666667 # [2,] 0.6666667 -0.3333333 solve(x)%*%x # [,1] [,2]

# [1,] 1 1.110223e-16

# [2,] 0 1.000000e+00

(Note the slight rounding error); whereas

(x^(-1))%*%x

# [,1] [,2] # [1,] 2.0 3.25 # [2,] 1.3 2.00

Best wishes,

Ted.

E-Mail: (Ted Harding) <Ted.Harding_at_manchester.ac.uk> Fax-to-email: +44 (0)870 094 0861

Date: 02-Mar-08 Time: 21:05:50 ------------------------------ XFMail ------------------------------ ______________________________________________R-help_at_r-project.org mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Sun 02 Mar 2008 - 21:13:57 GMT

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