[R] Question about syntax in lm function

[Ben Bolker]

阮铮 <rz1991 <at> foxmail.com> writes:

>  I encounters some codes in ggplot2 manual and confused with one of
> its lm syntax. 
[snip]

mod <- lm(lprice2 ~ lcarat * color, data = d) 

> # *** what puzzled me is the
> last statement marked with ***. How does R deal with lcarat * color,
> since color is not of numeric value. If this is ok, how can I write
> the mathematical formula of this regression model ?  

  See chapter 11 of the introduction to R.  This formula specification
fits what might be described as an ANCOVA model, where the jth individual
in the ith level of "color" has expected mean value

  mu_{ij} = a_i + b_i*lcarat_{ij}

That is, different slopes and intercepts for each level of 'color'.
That isn't exactly the way R parameterizes the model, but it should
get you started.
  You can look around for search results on "ANCOVA in R" to learn
more.