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Topic: Transformations
Replies: 1   Last Post: Oct 22, 1996 10:41 PM

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 James Lang Posts: 48 Registered: 12/6/04
Transformations
Posted: Oct 21, 1996 11:18 PM

>a student collects a set of data for which an approximate model might be:
>
> y = 3log(x - 40) + 30 or even y = 3(x-40)^2 + 30
>
>Am I right that there is no easy transformation which will "straighten" these
>"curves"??? If so what should we do with the data when trying to model it?

Al,
I think you are correct when the parameters are unknown. That is if the
function form is
y = a(x - h)^2 + k and a,h,k must be estimated from the data. This
also occurs for exponential functions with a vertical shift y = a(b^x) + c.

I would suggest a two step process.

1) Visually fit the data and use the coordinates of points on the
visual fit with algebra methods to get a first approximation to the
parameters. For example, in the case of the quadratic, given three points
you can find a parabola to fit the points.

2) Graph your fit, and calculate SSE = sum(yfit - ydata)^2.

This approach reviews some algebra and the concept of SSE.

James

Valencia Community College
Orlando, FL

Date Subject Author
10/21/96 James Lang
10/22/96 Paul Velleman