Trendlines

["Robert A. Ferrell" <ferrell1@home.com>]

Indubitably, there is a rising popularity with spreadsheet software
(i.e. Microsoft Excel and others).
Many such software provides options to add trendlines to your data
charts.

Trendlines:
Linear
Logarithmic
Polynomial
Power
Exponential
Moving Average

Is there anytime that a 'trendline' provides value to a time series
chart?
Or more specifically a control chart?
How helpful is the line in determining 'trends'?
Does a trendline help identify the cause?
What is the operational definition of 'trend'?
Does a rising or declining trend (in the sense of trendline) actually
mean there is a problem?

Learning always,
Robert A. Ferrell
mailto:ferrell1@home.com



The Following are excerpts from the Microsoft Excel 97 Help File:

Equations for calculating trendlines

Linear

Calculates the least squares fit for a line represented by the
following equation:
y = mx + b
where m is the slope and b is the intercept.

Polynomial

Calculates the least squares fit through points by using the following
equation:
y = b+ c1x + c2xx + c2xxx +...+ c6xxxxxx
where b and    are constants.

Logarithmic

Calculates the least squares fit through points by using the following
equation:
y = c1n x + b
where c and b are constants, and ln is the natural logarithm function.

Exponential

Calculates the least squares fit through points by using the following
equation:

y = ce(superscript bx)

where c and b are constants, and e is the base of the natural
logarithm.

Power

Calculates the least squares fit through points by using the following
equation:
y = cx(superscript b)
where c and b are constants.

Moving Average

Ft = At = At-1...+At-n=+1

Note   The number of points in a moving average trendline equals the
total number of points in the series less the number you specify for
the period.




Trendlines in charts

Trendlines are used to analyze problems of prediction. Such analysis
is also called regression analysis. By using regression analysis, you
can extend a trendline in a chart forward or backward beyond the
actual data to show a trend. You can also create a moving average,
which smooths out fluctuations in data and shows the pattern or trend
more clearly.
You can add trendlines to data series in unstacked 2-D area, bar,
column, line, stock, xy (scatter), and bubble charts. You cannot add
trendlines to data series in 3-D, stacked, radar, pie, or doughnut
charts. If you change a chart or data series so that it can no longer
support the associated trendline ¾ for example, by changing the chart
type to a 3-D chart ¾ you lose the trendlines.
=======================================================================