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Re: [Phys-L] Question on data analysis



i agree with the spirit of this. so in an ohm's law lab, make a point of
noting; when we applied a voltage of zero, our ammeter registered 0
amperes. Now, (0,0) is a data point. In a snell's law lab, measure the
angle of refraction when incidence was zero. Again, (0,0) is a data point.


On the other hand, sometimes I do think it is reasonable to suspect an
implied zero point. Earlier this year, my students did the lab where you
drop coffee filters to get terminal velocities. They dropped 1, 2, 3, 4,
and 5 filters. The graph of terminal velocity vs mass looks linear. And
you can't drop half a filter. But if you include a forced zero, you can
see that the relationship is not linear.

On Thu, May 21, 2015 at 12:26 PM, Dan Beeker <debeeker@comcast.net> wrote:

One should always honor their data. Why would you force it to go through
zero? That's backwards. If you have a model that predicts your data should
go through zero, then one should look for reasons why it doesn't.
Otherwise, why bother acquiring the data in the first place?

Dan Beeker

On May 21, 2015, at 12:00 PM, phys-l-request@www.phys-l.org wrote:

Date: Thu, 21 May 2015 08:50:17 +0000
From: Savinainen Antti <antti.savinainen@kuopio.fi>
To: "phys-l@phys-l.org" <phys-l@phys-l.org>
Subject: [Phys-L] Question on data analysis
Message-ID:
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03A5CDAD2EA0864F9A5E4157AC2EBD945A65E54C@ASGARD-2.istekkipalvelut.fi>
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Hi,

spreadsheet programs usually have a feature which allows to force a
trendline through the origin. Of course,
the same thing can be done manually as well. My question is: when it is
reasonable to force a trendline through the origin? I mean, a non-zero
y-xis intercept may have a physical interpretation or it may indicate
systematic error in measurements or (more realistically) may be a
combination of both. Given these, what could be a benefit for the forcing
of the trendline through the origin?

The question was motivated by an observation that a chemistry colleague
insists that her studenst must force a trendine through the origin (in case
there is no chemical interpretation, I suppose) for the y-axis intercept.

Regards,

Antti
Finland

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