Idential trend line equations show different trends

Hi

I have plotted a trend line based on some actual known data. I have then manually calculated the y figure by plugging in the trend line formula.

y = 5E-05x4 - 0.0283x3 + 5.4408x2 - 521.05x + 29719

On that attachment you can see a difference in results even though its the same trend line.

Any ideas?

Thanks

Wired

Wired PSF -

(1) Use considerably more significant digits (up to 15) in your
calculations. The first term, 0.00005*x^4, lacks precision.

(2) Avoid overfitting your data. A polynomial of order 4 is seldom
appropriate.

- Mike
http://www.mikemiddleton.com


"Wired PSF" <[email protected]> wrote
in message news:[email protected]...
>
> Hi
>
> I have plotted a trend line based on some actual known data. I have
> then manually calculated the y figure by plugging in the trend line
> formula.
>
> y = 5E-05x4 - 0.0283x3 + 5.4408x2 - 521.05x + 29719
>
> On that attachment you can see a difference in results even though its
> the same trend line.
>
> Any ideas?
>
> Thanks
>
> Wired
>
>
> +-------------------------------------------------------------------+
> |Filename: dummy data.gif |
> |Download: http://www.excelforum.com/attachment.php?postid=5103 |
> +-------------------------------------------------------------------+
>
> --
> Wired PSF
> ------------------------------------------------------------------------
> Wired PSF's Profile:
> http://www.excelforum.com/member.php...o&userid=36795
> View this thread: http://www.excelforum.com/showthread...hreadid=565052
>

Also try to check the t-values of each coefficients.

Try to use LINEST function.

Note: LINEST is a matrix function. You need to enter the formula by
ctrl-shift-enter rather than just enter. See the help function from
LINEST. It will tell you more about multivariable regressions.

Also note that office2k and office may give slightly different results if
your variables are inter-related.

Man







"Mike Middleton" <[email protected]> wrote in message
news:%[email protected]...
> Wired PSF -
>
> (1) Use considerably more significant digits (up to 15) in your
> calculations. The first term, 0.00005*x^4, lacks precision.
>
> (2) Avoid overfitting your data. A polynomial of order 4 is seldom
> appropriate.
>
> - Mike
> http://www.mikemiddleton.com
>
>
> "Wired PSF" <[email protected]> wrote
> in message news:[email protected]...
> >
> > Hi
> >
> > I have plotted a trend line based on some actual known data. I have
> > then manually calculated the y figure by plugging in the trend line
> > formula.
> >
> > y = 5E-05x4 - 0.0283x3 + 5.4408x2 - 521.05x + 29719
> >
> > On that attachment you can see a difference in results even though its
> > the same trend line.
> >
> > Any ideas?
> >
> > Thanks
> >
> > Wired
> >
> >
> > +-------------------------------------------------------------------+
> > |Filename: dummy data.gif |
> > |Download: http://www.excelforum.com/attachment.php?postid=5103 |
> > +-------------------------------------------------------------------+
> >
> > --
> > Wired PSF
> > ------------------------------------------------------------------------
> > Wired PSF's Profile:
> > http://www.excelforum.com/member.php...o&userid=36795
> > View this thread:
http://www.excelforum.com/showthread...hreadid=565052
> >
>
>