Hi,
I would explicitly warn the user.
The chart drawing engine should test, IF ALL(x) are numbers. IF this is
true, then a warning should be displayed. The user most probably wanted
these explicit x-values. [A possible exception to the previous comment
would be IF x stores a date. Despite x being a number - as dates are
basically stored as numbers in Calc - they are not supposed to be used
in the calculation of the regression line. But it would be difficult to
detect dates, unless a new data type "Date" would be introduced - and
NOT merely a 'display-like-date' flag.]
Sincerely,
Leonard
Regina Henschel wrote:
Hi Bjoern,
Bjoern Milcke schrieb:
Hi Regina,
Hello,
because there is no obvious solution, I'll bring this to discussion
here:
Example table
x y
-2 -4,84
-1 -1,74
0 0
1 2,38
2 3,82
3 7,17
4 9,28
gives for a linear regression the equitation (as shown in status bar)
y= 2.286 ∙ x − 6.847 for a line-diagram
y= 2.286 ∙ x + 0.01 for an XY-diagram
The wrong equitation in the line-diagram is due to the fact, that
for line diagrams not the real x-values but the values 1, 2, … are
used.
The question is now, whether the regression curve should do so and
it is enough, if we tell it to the user. Or should the calculation
use the x-values of the data series, if they provide a datatyp to
calculate with?
The problem here is that there is this fundamental difference in line
charts and scatter charts. A line chart uses categories (which are
strings) and has equidistant data points. The "x-values" in this
example are names for categories for all data series of a line chart.
If you want them to be x-values, you have to chose scatter as
chart-type.
I know that, but I see often users simple click type "line" to get a
chart with lines, with all the problems.
The data series does not know any x-values in a line chart
Why not? The data range is known so it should be possible to look
whether there are numbers.
, so neither
does the regression curve. In addition your example works only
because the x-values are by chance equidistant. If they weren't, a
line chart would show the data points still equidistant. A regression
curve using the non-equidistant x-values would simply be wrong (the
graphs would not fit).
The equitations are wrong in the most cases. The question is, where
the user should be told, that the shown equitation doesn't fit to
their data series numbers.
The only chance I see for this dilemma is to guide the user somehow
that he should use a scatter chart when the "categories" are numbers.
I can suggest some text for the online help (would be issue 77929,
help file /text/schart/01/04050100.xhp), that's no problem. But will a
user with small mathematical knowledge notice, that something is wrong
and then look into the help? A warning when creating the chart would
be more helpful.
In
Excel there is an automatism that uses scatter charts even if you
select a line chart type, when the categories are all numbers.
In my Excel97 and Excel2007 there is no such automatism. Excel creates
the line chart without warning and shows the "wrong" equitation in the
chart.
However, I would
prefer to make this clear to the user.
Shall I write an issue for displaying a warning, when creating the
chart or do you think it is enough to add an explanation to the help?
kind regards
Regina
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