In a [previous post](http://blog.dotphys.net/2008/09/basics-making-graphs-with-kinematics-stuff-part-ii/), I talked about how to plot kinematics data with a spread sheet and how to fit a quadratic function to the data. In the back of my head I remember “Don’t trust Excel”. I seem to recall someone claiming that Excel did not do a proper fit. To test this, I collected some data and used several methods to fit the data:
- MS Excel’s built in function fitting
- Using the spread sheet (Excel) to manually calculate the best fit parameters
- Vernier’s Logger Pro (version 3.6.1)
- Plot 0.997 – http://plot.micw.eu/ – a program derived from Sci-Plot
I already discussed how to add a quadratic fit in Excel using the built in tools. Perhaps later I will also discuss Logger Pro and Plot. But how do you come up with a function to fit data? The basic idea is to create a quadratic function and vary the parameters such that the deviation of the actual data from the function is minimized. That is much detail as I want to go into except for the following two links that I used:
- A good description of Least Squares Fit to Quadratic Data
- Part of a least squares fit involves solving three equations with three unknowns. Here is a description of using Excel’s Solver to do this
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