[R] formula

[Adaikalavan Ramasamy]

If you want to fit "y = a + bx", then you use "lm(y ~ x)" instead of "lm(y ~ A + bx)". 
'A' is not a parameter but coefficient and you do not need to specify coefficients, 
which is what the linear model is trying to do anyway !
See the details section of help("formula").

> x <- 1:5
> y <- c(0, 1.0, 1.7, 2.0, 2.1)
> lm(x ~ y)

Call:
lm(formula = x ~ y)

Coefficients:
(Intercept)            y
     0.6828       1.7038

If A was already defined, and you are trying to multiply the y-values, then use the I() operator.
This protects the term/calculation and inhibits the usual term interpretation in linear models.

> lm(x ~ I(2*y) )

Call:
lm(formula = x ~ I(2 * y))

Coefficients:
(Intercept)     I(2 * y)
     0.6828       0.8519

PS : I think there is a typo in the y input as there is no such number as 2.1.4




On Thu, 2004-07-15 at 16:28, solares at unsl.edu.ar wrote:
> Hi, i 'dont understand how to take a general formula, view this:
> 
> x<-1:5
> y<-c(0,1,1.7,2,2.1.4)
>  dummy<-data.frame(x=x,y=y)
> formula<-"y~A*log(x)/log(2)"
> formu<-as.formula(formula)
> fm<-lm(formu,data=dummy)
> Error in eval(expr, envir, enclos) : Object "A" not found
> 
> but A is the parameter of fitting, why is this?Thanks Ruben
> 
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