[R] testing diff for slopes and intercepts

[Bill Simpson]

I fit the model

fit<-lm(thresh~cond*Ne)
where
thresh is the reponse
cond is a factor with levels a, b, and c
Ne is a continuous indep var

I think of this full model as having three lines: thresh as a function of
Ne for each condition. Thus we have slopea, slopeb, slopec, inta, intb,
intc.
lm output       my params
-------------------------
(Intercept)   inta  
condb         intb - inta
condc         intc - inta
Ne            slopea
condb:Ne      slopeb - slopea
condc:Ne      slopec - slopea

This parametrization automatically compares
inta vs intb
inta vs intc
But how to get test for diff of intb vs intc?

Same question for slope: I already have
slopea vs slopeb
slopea vs slopec
But how to get test for diff of slopeb vs slopec?

Thanks for any help.

Bill Simpson

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