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[Phys-L] Re: Charts or Graphs, and other Excel stuff



I'm sorry, but people are totally misunderstanding what I am saying.
From my viewpoint this has never been a discussion about how to make
Excel or any other spreadsheet work. My point has always been about
"presentation." How are the students presenting the results in their
reports?

I know perfectly well how to get the intercept value from a
linear-regression fit of data using Excel. You can use the LINEST that
Denker mentioned (which is the hardest way to get it). You can use the
INTERCEPT function (which is the easiest way to get it). Or you can
make a scatter plot and fit a linear trendline then use the "Display
equation on chart" option. I think this third method is the best way to
get it assuming you also want the graph... er... chart. Why do extra
work? Get the graph, trendline, and trendline parameters all from
within the graph settings.

I also know how to FORECAST the linear regression line on the graph
beyond the data points so the drawn trendline extends to an abscissa
value of zero even when all data points have abscissa values greater
than zero.

I've known all this from the beginning. You don't need to tell me how
to use Excel. I consider myself an Excel expert and many faculty come
to me for Excel advice. I am not fighting Excel. I am fighting
students.

For example, concerning the intercept, I am not saying the students
aren't calculating the intercept. I am not saying the students are not
displaying the intercept value somewhere on the graph or in the report.
What I am saying is Excel is not automatically extending the visual
regression line on the graph to an abscissa value of zero. That means
Excel is not visually showing the linear-regression line hitting or
crossing the vertical axis.

It seems to me that if the intercept is an important value that the
scientist wants the reader to notice, and if a graphical presentation of
the data is going to be produced, and if the scale is such that the
displayed vertical axis has an abscissa value of zero, then good
graphing etiquette would be to allow the viewer to see the
linear-regression line hitting or crossing the vertical axis rather than
ending the regression line at the first data point.

For my use of the words "good graphing etiquette," please interpret this
in the following way... A graph is used to visually communicate
something to the viewer. Good graphing etiquette implies the graph is
designed/presented in a manner that allows the viewer to get the desired
communication quickly, effortlessly, and correctly.

I am saying that typical spreadsheet software does not always
automatically follow what I consider to be good graphing etiquette.
After the software has done its default thing, the default graph has to
be tweaked to turn it into a better communication tool. I know that
Excel and Quattro Pro have the necessary tools to modify the default
graph into a better communication. My complaint is that students settle
for the default format and don't take the time to think about whether
the graph needs improvement.

When I harp at them about this, they typically come back with comments
like, "Well if that's the way a graph is supposed to look, why didn't
Excel do it that way." I reply that Excel can't read their minds about
what they are trying to communicate. Excel is only an analysis and
communication tool. The final result has to be tailored by the
scientist to yield the desired communication.

Okay, if that's my problem, then why did I bring this up as a
spreadsheet discussion? Because as we have given students computer
tools to make their data analysis and data presentation easier, I am
afraid we have also turned off their brains. There is something to be
said about drawing graphs by hand. The whole time you are laying it out
by hand you have to think about things like... How big should this graph
be? How should I space out and label the scales on the axes? When I
draw the regression line, where should I start it and where should I
stop it? A student MUST make these decisions when graphing by hand.
You simply cannot draw the graph without making these decisions. A
student does not have to make any of these decisions to "get a graph"
out of a spreadsheet program. When they don't have to think, they don't
think.

Whether it is called a graph or a chart is a minor thing. My major
point is that spreadsheet software, as wonderful as it is, has perhaps
had some negative effects. It gives students the illusion they don't
have to make decisions about how the graph should look, or how the data
in a table should look. It gives them the illusion the software will
take care of all that for them. I am saying it does not. The default
output is only a starting point. And, since spreadsheet software is
designed for business use, the default result might be further from the
desired result for a scientific application than it would be for a
business application.

I may have mentioned that my daughter is currently pursuing a graphic
design degree. The things I am talking about are what graphic designers
have to do all the time. I am pleased that she tells me her program
makes some use, but little use, of graphic-design software during the
first two years of the program. They are drawing and coloring by hand
and constructing 3D objects by hand. The graphic design profs don't
want computer software making any decisions for the students. They
don't want the students to rely on software until they understand the
graphic communication process. I think that's a smart program. The
fact that scientists are not graphic designers does not excuse us from
having to make graphic-design decisions. The graphic result we want is
often different from or beyond what the typical business-oriented
spreadsheet produces by default. Are we teaching the necessary things
to our students?

Some have mentioned that we shouldn't be too particular about this
because we don't want the details to get in the way of learning the
physics. It's even been called "anal retentive." I had a wise physics
teacher that often said, "You haven't learned anything until you have
communicated it." If we scientists can't communicate our knowledge to
ourselves and others, what good is our knowledge? Effective
communication must be a part of our science curriculum.

Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu