A easy way to write formula
On Sun, Jul 27, 2008 at 4:19 AM, Mark Difford <mark_difford at yahoo.co.uk> wrote:
Hi Jinsong and Thierry,
(x1 + x2 + x3) ^2 will give you the main effects and the interactions.
Although it wasn't specifically requested it is perhaps important to note that (...)^2 doesn't expand to give _all_ interaction terms, only interactions to the second order, so the interaction term x1:x2:x3 will be missing, To get these, do (x1 + x2 + x3)^3
Or just x1 * x2 * x3 Hadley