[R] investigating interaction term for a model of Gross Primary Productivity

[John Fox]

Dear Stephen,

I'm not sure exactly what you have in mind, but you might take a look
at the effects package.

I hope this helps,
 John

On Wed, 15 Oct 2008 12:59:33 -0400
 "stephen sefick" <ssefick at gmail.com> wrote:
> I am trying to investigate the interaction term in the below.  The
> paradigm in aquatic systems is that algal production is either
> nitrogen (TIN) or Phosphorus limited, and I am trying to investigate
> this-  what is the best way to go about investigating the interaction
> term.  I have some thoughts on the above, but I will withhold them to
> see what others think.  Thanks for your help.
> 
> d <- (structure(list(d.GPP = c(1.213695235, 3.817313822, 1.267930498,
> 10.45692825, 3.268295623, 3.505286001, 4.468225245, 0.915653726,
> 1.635617261, 3.726133898, 1.363453706, 13.99650967, 0.417618143,
> 0.741080504, 0.412440872, 3.515743675, 8.248491445, 1.537773727,
> 1.537773727, 3.249103284, 3.249103284, 0.768531626, 2.633107621,
> 3.113199095, 0.773824094, 0.680150068, 0.680150068, 0.026385752,
> 0.369310858, 8.049276658, 7.487378383, 0.950072035, 0.950072035,
> 0.763580377, 0.333244629, 5.475999014, 9.235631398), d.TIN = c(0.53,
> 0.52, 0.446, 0.217, 0.39, 0.34, 0.45, 0.29, 0.23, 0.308, 0.3,
> 0.1, 0.52, 0.13, 0.38, 0.36, 0.226, 0.345, 0.345, 0.217, 0.217,
> 0.47, 0.34, 0.31, 0.35, 0.41, 0.41, 0.432, 0.333, 0.168, 0.22,
> 0.37, 0.37, 0.44, 0.38, 0.3, 0.105), d.Phosphorus = c(0.17, 0.19,
> 0.2, 0.012, 0.13, 0.14, 0.069, 0.13, 0.13, 0.13, 0.099, 0.013,
> 0.16, 0.076, 0.12, 0.037, 0.011, 0.1, 0.1, 0.021, 0.021, 0.12,
> 0.099, 0.088, 0.036, 0.15, 0.15, 0.12, 0.12, 0.0076, 0.014, 0.14,
> 0.14, 0.15, 0.15, 0.12, 0.011), d.TSS = c(2.8, 8.4, 11, 1.3,
> 4.2, 2, 3.4, 14, 8.2, 3.1, 1.4, 0.9, 6.1, 9.2, 11, 1.2, 1.3,
> 11, 11, 8.5, 8.5, 13, 4.4, 1.4, 2.1, 25, 25, 9.3, 6.1, 1.6, 1.5,
> 19, 19, 24, 9.6, 1.8, 1.4)), .Names = c("d.GPP", "d.TIN",
> "d.Phosphorus",
> "d.TSS"), row.names = c(NA, -37L), class = "data.frame"))
> 
> #here is the model
> model.GPP <- lm(GPP~TIN*Phosphorus+I(1/TSS), data=d)
> 
> #regression equation
> GPP <- function(TIN, P, TSS){
> 	8.765-(22.896*TIN)-(63.513*P)+(5.836/TSS)+(170.735*(TIN*P))
> }
> 
> #predicting GPP from the data - This is kind of useless in this
> context because I am predicting GPP from the data that I modeled it
> with, but I think I am going to have to fool around #with these
> values
> to investigate the interaction term.  This is kind of where I am
> stuck.
> a <- GPP(d$TIN, d$Phosphorus, d$TSS)
> 
> #plotting the data
> plot(a~d$TSS)
> 
> 
> -- 
> Stephen Sefick
> Research Scientist
> Southeastern Natural Sciences Academy
> 
> Let's not spend our time and resources thinking about things that are
> so little or so large that all they really do for us is puff us up
> and
> make us feel like gods.  We are mammals, and have not exhausted the
> annoying little problems of being mammals.
> 
> 								-K. Mullis
> 
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--------------------------------
John Fox, Professor
Department of Sociology
McMaster University
Hamilton, Ontario, Canada
http://socserv.mcmaster.ca/jfox/