Re: [R] Least Squares Fit

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From: Martin Maechler (maechler@stat.math.ethz.ch)
Date: Mon 03 May 2004 - 20:21:47 EST


Message-id: <16534.7483.243268.790775@gargle.gargle.HOWL>


>>>>> "Jon" == Jonathan Baron <baron@psych.upenn.edu>
>>>>> on Sun, 2 May 2004 07:10:10 -0400 writes:

    Jon> On 05/02/04 03:26, Fred J. wrote:
>> Hello
>>
>> I need to plot the least squares fit and get the slope of
>> the line that best fit the data. after reading lm and
>> lsfit, since not being able to understand the use of the
>> second argument "y" for the lsfit, I am giving the lm a
>> go, but know not why the code below does not draw the
>> line on the plot. e.g. well.. why -50 Intercept? thanks
>>
>> x <- 1:10; y <- x+50
>> plot(x,y)
>> z <- lm(as.data.frame(cbind(x,y))))
>> abline(z)
>>
>>> z$coefficients
>> (Intercept) y
>> -50 1

Did you, "Fred J.", really ever take time to read the
"Introduction to R", maybe with a pencil in your hand to mark a
few things?

    Jon> The lm() is predicting x from y, and plot() is plotting
    Jon> y as a function of x.

    Jon> Try
    Jon> plot(x,y)
    Jon> abline(lm(y~x))

or in this case, the more intuitive

     plot(y ~ x)
abline(lm(y ~ x))

y ~ x : `` y is modeled by (predictor) x ''

--
Martin

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