linear trend lines

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bonzo - 09 Dec 2003 17:58 GMT

can anyone help?
I have to put 95% confidence intervals around a linear trend line. How
do I work out the values for the trend line? I am using a scatter
graph with 2 sets of data.
Please help!

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Jerry W. Lewis - 10 Dec 2003 05:01 GMT

The confidence bound for the estimated line at x is

=FORECAST(x,known_y's,known_x's) +/- TINV((1-conf)*2,COUNT(known_y's)-2)
*STEYX(known_y's,known_x's)
*SQRT(1/COUNT(known_y's)+(x-AVERAGE(known_x's))^2/DEVSQ(known_x's))

the confidence bound for an individual observation at x is

=FORECAST(x,known_y's,known_x's) +/- TINV((1-conf)*2,COUNT(known_y's)-2)
*STEYX(known_y's,known_x's)
*SQRT(1+1/COUNT(known_y's)+(x-AVERAGE(known_x's))^2/DEVSQ(known_x's))

Jerry

> can anyone help?
> I have to put 95% confidence intervals around a linear trend line. How
> do I work out the values for the trend line? I am using a scatter
> graph with 2 sets of data.
> Please help!

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Gary Klass - 14 Dec 2003 14:54 GMT

bonzo:
If Jerry's solution may be too complicated for you, try this:
Run the regression program in Data Analsysi add-in.

use the "Standard Error" (of the estimate, underneath the R-square).

Calculate two new variables plus or minus 2 standard errors. and plot them.

> The confidence bound for the estimated line at x is
>
[quoted text clipped - 15 lines]
>> graph with 2 sets of data.
>> Please help!

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