Hi, Sorry by this newbie stat doubt, but with some brains in the list anybody can help-me. I have a sistem composed by 3 quantitative variables. The variables are: The growth (Y in cm) The nutrient (X1 in gram) The water (X2 in ml) Ok, My doubt is: Is correct or necessary make a iterations of the nutrient and water, or the iteraction is intrinsical of the model. glm(growth~nutrient+water) or glm(growth~nutrient+water+nutrient:water) Working with one quantitative explanatory variable e one qualitative explanatory variable (with some levels), I understandy the iteration of quantitative x qualitative, for testing the curve inclination. But when I have some quantitative explanatory variables, the iteration generate a new vector by x1*x2 and the number generated by this vector is no sense (30g*2ml=60 and 2g*30ml=60, but is very different). I need to make iteraction or not? A less important question: How to make a plot with this and a curve of this? the curve function use only one x, I have two or more x in same function. Thanks and sorry again Ronaldo -- His mind is like a steel trap: full of mice. -- Foghorn Leghorn -- | //|\\ [*****************************][*******************] || ( ? ? ) [Ronaldo Reis J?nior ][PentiumIII-600 ] | V [ESALQ/USP-Entomologia, CP-09 ][HD: 30 + 10 Gb ] || / l \ [13418-900 Piracicaba - SP ][RAM: 128 Mb ] | /(lin)\ [Fone: 19-429-4199 r.229 ][Video: SiS620-8Mb ] ||/(linux)\ [chrysopa at insecta.ufv.br ][Modem: Pctel-onboar] |/ (linux) \[ICQ#: 5692561 ][SO: CL 7.0 (2.2.19)] || ( x ) [*****************************][*******************] ||| _/ \_Powered by Conectiva Linux 7.0 D+:) | Lxuser#: 205366 -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.- r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html Send "info", "help", or "[un]subscribe" (in the "body", not the subject !) To: r-help-request at stat.math.ethz.ch _._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._