The offset(x) term means to include x (or whatever is in the parentheses)
into the model as is, without computing a slope for that term.  You could
also include offset( 1 * x ) instead which might make this a bit more
explicit (but would not actually make any difference).  Since x by itself
is also in the model with a computed slope that slope will measure the
difference from 1.  So the model is fitting y = b0 + b1 * x + 1 * x + e
with b0 and b1 being computed, if we factor that then it is y = b0 + (b1 +
1) * x + e, so the estimate of b1 is how much the overall slope of x
differs from 1 and the standard test on b1 now does what you want.


On Sat, May 4, 2013 at 5:35 AM, Elaine Kuo <elaine.kuo.tw@gmail.com> wrote:

> Hello,
>
> Thanks.
> But the parameter offset is new to me.
> Please kindly explain why setting offset to x will give a significant test
> of whether the slope coefficient is different from one.
> (I checked the ?lm but still do not understand it well)
>
> Thanks again
>
> Elaine
>
>
> On Wed, May 1, 2013 at 11:12 AM, Thomas Lumley <tlumley@uw.edu> wrote:
>
> > Or use an offset
> >
> > lm( y ~ x+offset(x), data = dat)
> >
> > The offset gives x a coefficient of 1, so the coefficient of x in this
> > model is the difference between the coefficient of x in the model without
> > an offset and 1 -- the thing you want.
> >
> >     -thomas
> >
> >
> > On Wed, May 1, 2013 at 2:54 PM, Paul Johnson <pauljohn32@gmail.com>
> wrote:
> >
> >> It is easy to construct your own test. I test against null of 0 first
> so I
> >> can be sure I match the right result from summary.lm.
> >>
> >> ## get the standard error
> >> seofb <- sqrt(diag(vcov(lm1)))
> >> ## calculate t. Replace 0 by your null
> >> myt <- (coef(lm1) - 0)/seofb
> >> mypval <- 2*pt(abs(myt), lower.tail = FALSE, df = lm1$df.residual)
> >>
> >> ## Note you can pass a vector of different nulls for the coefficients
> >> myt <- (coef(lm1)  - c(0,1))/seofb
> >>
> >> We could write this into a function if we wanted to get busy.  Not a bad
> >> little homework exercise, I think.
> >>
> >>
> >>
> >>
> >> > dat <- data.frame(x = rnorm(100), y = rnorm(100))
> >> > lm1 <- lm(y ~ x, data = dat)
> >> > summary(lm1)
> >>
> >> Call:
> >> lm(formula = y ~ x, data = dat)
> >>
> >> Residuals:
> >>     Min      1Q  Median      3Q     Max
> >> -3.0696 -0.5833  0.1351  0.7162  2.3229
> >>
> >> Coefficients:
> >>              Estimate Std. Error t value Pr(>|t|)
> >> (Intercept) -0.001499   0.104865  -0.014    0.989
> >> x           -0.039324   0.113486  -0.347    0.730
> >>
> >> Residual standard error: 1.024 on 98 degrees of freedom
> >> Multiple R-squared: 0.001224,    Adjusted R-squared: -0.008968
> >> F-statistic: 0.1201 on 1 and 98 DF,  p-value: 0.7297
> >>
> >> > seofb <- sqrt(diag(vcov(lm1)))
> >> > myt <- (coef(lm1) - 0)/seofb
> >> > mypval <- 2*pt(abs(myt), lower.tail = FALSE, df = lm1$df.residual)
> >> > myt
> >> (Intercept)           x
> >> -0.01429604 -0.34650900
> >> > mypval
> >> (Intercept)           x
> >>   0.9886229   0.7297031
> >> > myt <- (coef(lm1) - 1)/seofb
> >> > mypval <- 2*pt(abs(myt), lower.tail = FALSE, df = lm1$df.residual)
> >> > myt
> >> (Intercept)           x
> >>   -9.550359   -9.158166
> >> > mypval
> >>  (Intercept)            x
> >> 1.145542e-15 8.126553e-15
> >>
> >>
> >> On Tue, Apr 30, 2013 at 9:07 PM, Elaine Kuo <elaine.kuo.tw@gmail.com>
> >> wrote:
> >>
> >> > Hello,
> >> >
> >> >
> >> >
> >> > I am work with a linear regression model:
> >> >
> >> > y=ax+b with the function of lm.
> >> >
> >> > y= observed migration distance of butterflies
> >> >
> >> > x= predicted migration distance of butterflies
> >> >
> >> >
> >> >
> >> > Usually the result will show
> >> >
> >> > if the linear term a is significantly different from zero based on the
> >> > p-value.
> >> >
> >> > Now I would like to test if the linear term is significantly different
> >> from
> >> > one.
> >> >
> >> > (because I want to know if the regression line (y=ax+b) is
> significantly
> >> > from the line with the linear term =1 and the intercept =0)
> >> >
> >> >
> >> >
> >> > Please kindly advise if it is possible
> >> >
> >> > to adjust some default parameters in the function to achieve the goal.
> >> >
> >> > Thank you.
> >> >
> >> >
> >> > Elaine
> >> >
> >> >         [[alternative HTML version deleted]]
> >> >
> >> > ______________________________________________
> >> > R-help@r-project.org mailing list
> >> > https://stat.ethz.ch/mailman/listinfo/r-help
> >> > PLEASE do read the posting guide
> >> > http://www.R-project.org/posting-guide.html
> >> > and provide commented, minimal, self-contained, reproducible code.
> >> >
> >>
> >>
> >>
> >> --
> >> Paul E. Johnson
> >> Professor, Political Science      Assoc. Director
> >> 1541 Lilac Lane, Room 504      Center for Research Methods
> >> University of Kansas                 University of Kansas
> >> http://pj.freefaculty.org               http://quant.ku.edu
> >>
> >>         [[alternative HTML version deleted]]
> >>
> >> ______________________________________________
> >> R-help@r-project.org mailing list
> >> https://stat.ethz.ch/mailman/listinfo/r-help
> >> PLEASE do read the posting guide
> >> http://www.R-project.org/posting-guide.html
> >> and provide commented, minimal, self-contained, reproducible code.
> >>
> >
> >
> >
> > --
> > Thomas Lumley
> > Professor of Biostatistics
> > University of Auckland
> >
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>



-- 
Gregory (Greg) L. Snow Ph.D.
538280@gmail.com

	[[alternative HTML version deleted]]

