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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