# Hypothesis

#### Mikkelsoeren

##### New Member
If you got this model:

mode 1) y=a +b1x1 + b2x2 + u

and this:

mode 2) y=a + b1(x1-x2) + u

What do you call the restriction there has been put on model 1 in model 2?

Is it just b1=-b2 ?
if yes, how do you test this kind of restriction? I really dunno:S

#### Dason

##### Ambassador to the humans
You could test the hypothesis:

Ho: b1 = -b2
Ha: b1 != -b2

through the use of contrasts. You could also do it through a likelihood ratio test with model 1 being the full model and model 2 being the reduced model.

#### Mikkelsoeren

##### New Member
yes okay, but i really dont know how to test that kind of hypothesis.
Is it possible you can provide me with a example?

#### Dason

##### Ambassador to the humans
What software are you using? Most packages have an easy way to specify a contrast.

#### Mikkelsoeren

##### New Member
I use Stata. Really hope you can help. I need to deliver an answer tomo:S

#### Dason

##### Ambassador to the humans
I've never used Stata so I can't help there.

#### Mikkelsoeren

##### New Member
nop okay. But can you tell me the theory i should do, point for point?

#### Mikkelsoeren

##### New Member
Okay friends, this is my exam. I did not think it was necessary to read up on the material because I thought I knew everything. I thought I got all knowledge from heaven

#### bryangoodrich

##### Probably A Mammal
What software are you using? Most packages have an easy way to specify a contrast.
I'd like to see the R way.

#### Dason

##### Ambassador to the humans
Well there is a package called "contrast" in R but I typically just do them by hand.

Code:
test.contrast = function(lm.out, C, d = 0){
# Provides a test of Ho: C*b = d vs. Ha: C*b != d
# lm.out: - is the linear model used
# C: ----- is a matrix with the desired set of contrasts
#          which may contain more than one row
# d: ----- a vector of values to test against

b <- coef(lm.out)
V <- vcov(lm.out)
df.numerator <- nrow(C)
df.denominator <- lm.out\$df
Cb.d <- (C %*% b) - d
Fstat <- drop(t(Cb.d) %*% solve(C %*% V %*% t(C)) %*% Cb.d/df.numerator)
pvalue <- 1 - pf(Fstat, df.numerator, df.denominator)
ans <- list(Fstat = Fstat, pvalue = pvalue)
return(ans)
}

# Generate some fake data
n <- 20
sigma <- 1
betas <- c(7, 2, -2)
x1 <- 1:n
x2 <- runif(n)
X <- matrix(c(rep(1,n),x1, x2), ncol = 3)
y <- X %*% betas + rnorm(n, 0, sigma)

# Fit the linear model
o <- lm(y ~ x1 + x2)

# Test the idea that b1 = -b2  ie b1 + b2 = 0
C <- matrix(c(0, 1, 1), nrow = 1)
test.contrast(o, C)

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#### Mikkelsoeren

##### New Member
dudes, im not much cleaver here. Please introduce me to the rapidhole

#### Dason

##### Ambassador to the humans
Okay friends, this is my exam. I did not think it was necessary to read up on the material because I thought I knew everything. I thought I got all knowledge from heaven
What? You thought you didn't have to read up on anything and that knowledge just came from heaven? Well you're wrong and you should probably start reading up on things...

dudes, im not much cleaver here. Please introduce me to the rapidhole
What are you trying to say here? Rapidhole? What?

Are you just asking for quick answers? You could try googling "linear contrast".

#### Mikkelsoeren

##### New Member
Dason :=) you are right. With a probability in 100%

#### bryangoodrich

##### Probably A Mammal
Isn't there some way to deal with the contrasts in the lm parameter list?

#### Dason

##### Ambassador to the humans
I've always thought that was only for certain types of contrasts though. I don't know how to specify a generic contrast through that parameter.

#### Mikkelsoeren

##### New Member
im macbook air cannot handle these calcuations... please help with an answer.

#### bryangoodrich

##### Probably A Mammal
I've always thought that was only for certain types of contrasts though. I don't know how to specify a generic contrast through that parameter.
I don't know. I've never really dealt with contrasts before. I know you can define some predefined functions on how to deal with contrasts, but the parameter in lm takes,

A list, whose entries are values (numeric matrices or character strings naming functions) to be used as replacement values for the contrasts replacement function and whose names are the names of columns of data containing factors.
I think I recall reading something somewhere about how you can define your own contrast functions, too, but maybe I'm thinking of something else.

#### Dason

##### Ambassador to the humans
Yeah the only thing I think I've seen that used for is to change from the R style contrasts to the SAS style contrasts when somebody was trying to get the R output to match up exactly with the SAS output.