Ploynomial Trendline Equation Changes After Copy Paste

[Guest]

In Excel 2007 if you create a XY Scatter Plot and then add a polynomial
trendline to the plot, display the equation, then try and copy/paste the
chart to a Word 2007 document the trendline equation in the chart drops one
of the orders. If you right click and select Format Trendline, it still
lists the correct order equation, but it does not show. If you delete the
trendline, and then add it again in the pasted copy of the chart the problem
is fixed. I have reproduced this using 2,3,4,5, and 6th order polynomial
equations.

 

[Bernard Liengme]

I was unable to reproduce this behaviour. Can you give more details?
best wishes

 

[Guest]

Sure....use this data

x y
1252.531345 44.0462161
1399.141231 54.03976206
1400.056416 44.5415568
1398.717824 36.45095719
1121.321671 69.02084478
1120.421493 57.35527023
1121.880135 58.10791134
1015.828469 52.80347407
842.7187937 73.74794476
755.8568419 69.50308062
841.0183109 68.54248481
560.0202985 81.17169689
503.6105019 79.47102425
560.4900412 81.38340795
281.0801761 94.02451592
252.2988083 96.7705903
281.3767037 92.81489618
0 100

Plot that data on an XY Scatter Plot in Excel 2007.

Select the series, then right click and "Add Trendline"

Use the following properties for the trendline:

Trend/Regression Type:
Polynomial - Order 3

Trendline Name:
Automatic

Check Set Intercept = 100
Check Display Equation on chart
Check Display R-squared value on chart

You will get an equation:
y = 6E-09x3 - 3E-05x2 - 0.021x + 100

Now, copy the chart and paste it into another Excel 2007 document, Word
2007, Powerpoint 2007, ect. document and note the equation. It will change
to the following:
y = - 3E-05x2 - 0.021x + 100

This is the previous equation minus the x^3 term. If you check the
Trendline properties, they are all still set as they where in the original
chart. The only way to get the equation working correctly again is to remove
then readd the trendline.

I hope you can reproduce this error, this occures on all of our Office 2007
installations here, Pro Plus and Standard about 40 copies. If there are
others in the engineering world experiencing this problem, and not noticing
it, there could be serious concequences for the projects they are working on.

 

[Guest]

For the data you specified, the cubic coefficient should be 9.6E-09, not
6E-09. IF you are not copying the entire coefficient, that may be why Word
is dropping it. If the chart itself is failing to display the correct value,
that would be another problem altogether.

Note that you can calculate the coefficients yourself, using LINEST. Select
three adjacent cells in one row and array enter
=LINEST(ydata-100,xdata^{1,2,3},FALSE)
That will give you the cubic, quadratic, and linear coefficients for the
cubic polynomial with intercept forced to be 100.

Jerry

 

[Guest]

Jerry,

This concerns me even more then. I am not missing any of the function when
I copy and paste, we are just literally copying the whole chart, which
includes the funciton, and pasting that in any other file, even the same
excel document again. I only copied the equation out seperatly to highlight
the problem I had noticed, which is that when you copy and paste the entire
chart it drops the cubic coefficient for absolutly no reason. Now if what
you are pointing out is correct, then Excel is not even computing the
Trendline equation correctly in the first place, that is even a bigger
problem. Can you verify the errors by following my steps?

 

[Guest]

I don't have 2007, but I have verified that both the chart trendline and
LINEST get correct results in 2000 and 2003. In versions prior to 2007, you
could right click on the chart equation and reformat to display scientific
notation with 14 decimal places; can that still be done in 2007? In older
versions that gave
y = 9.60309082652360E-09x3 - 2.63556800917837E-05x2 - 2.12916809517409E-02x
+ 1.00000000000000E+02
which are correct to between 12 and 14 significant figures.

Thus far, you have given no evidence of a problem with the calculated linear
and quadratic coefficients. If they are right, then the cubic coefficient is
probably right internally as well (since the values are inter-related). That
would suggest a problem with displaying the equation correctly, as opposed to
a problem calculating it correctly.

Jerry

 

[Guest]

Jerry,

Ok that explains it. We have been working with this fine in all previous
versions of Excel, it is just 2007 that shows this problem. If I reformat to
show scientific notation, this is the equation Excel 2007 gives me:

y = 5.95241766790664E-09x3 - 2.63556802117476E-05x2 - 2.12916808943767E-02x
+ 1.00000000000000E+02

If I do the same thing in Excel 2003, I get your result

y = 9.60309088524755E-09x3 - 2.63556802117502E-05x2 - 2.12916808943753E-02x
+ 1.00000000000000E+02

This just denfitly shows there is something screwy in the new version.

 

[Guest]

Extremely strange! Your cubic coefficient matches to 8-figures the cubic
coefficient for the model with an estimated intercept! I can think of no
obvious way for MS to have gotten things crossed up like this.

Unfortunately, this is not the only "improvement" in the 2007 chart
trendline that has rendered this formerly numerically excellent tool no
longer trustworthy
http://groups.google.com/group/microsoft.public.excel.charting/msg/67353c068ee07b94

I think that LINEST 2007 will still return correct values, using the
arguments that I posted earlier. Try generating the trendline from LINEST
coefficient estimates and see how it compares to the curve plotted by adding
a trendline to the chart (i.e. did MS mess up the displayed equation only, or
the plotted curve also?)

Jerry

 

[Guest]

Jerry,

Thanks so much for helping me out with this. I have made some calls and
hopefully raised some alarms at Microsoft about this. This is a MAJOR
inconsistency. I ended up taking the equation spit out by Excel 2007 and
using that to draw another line on my chart, and guess what, it does not
match the line drawn by the trendline function. For fun, I also added the
equation generated by Excel 2003, and it matches perfectly. Hopefully nobody
at NASA is using Excel 2007 to plot space ship trajectories. I will keep the
post updated with any further developments.

Chuck

 

[Bernard Liengme]

Yes, I see what you mean.
BTW: the problem also occurs when you copy to the SAME Excel file
I get the same coefficients with LINEST as reported by Jerry
Sorry to reply so late

 

[Guest]

Here is another thread reporting yet another new bug with the chart trendlin
http://groups.google.com/group/microsoft.public.excel.charting/browse_frm/thread/e3752db0fedc2cbf
With all the statistical issues that have needed attention for over 15
years, one wonders why MS started by "fixing" something that wasn't broken.

Jerry

 

[Guest]

No problem...I am just glad you where able to duplicate the problem. I also
found out the problem occurs also if you just save the Excel file and reopen
it....poof, missing coefficients.