In research experiments or the design exercises outlined opposite,
you probably need to fit a curve to a large number of data points.
How you do this depends partly on where your data is. As I’ve
said many times before in this column (because it’s true!),
for most design engineers this will be Excel. Excel itself includes
the capability to do a basic least squares fit: just plot your
data as an XY from the Chart menu. But most of us are likely to
need rather more powerful curvefitting to some
robust
statistical analysis capabilities, preferably without having to open
a separate statistics program.
If you use one of the high-end technical graphing packages, such
as (www.spss.com/sigmaplot) or Tecplot (www.tecplot.co.uk), you
can work with Excel data and perform many graphing operations as
well as curve-fitting. If, like many design engineers, you use
the powerful number-crunching capabilities of buy a Curve-Fitting
Toolbox which provides tools for fitting, analysing and managing
models. But if all you want to do is perform some curve-fitting
spreadsheet data, perhaps create a straightforward predictive model,
these are expensive solutions. So what else is available for fitting
curves to spreadsheet data and analysing the results?
Dr Know has
only been able to find to preview, build and customise charts in
the same Excel workbook where you store your experimental data.
That’s XLfit (www.adeptscience.co.uk/xlfit). It boasts a
wide range of 2D and 3D charts and extensive statistical analysis
tools, all accessible from within your Excel worksheet so you don’t
have to switch fitting and spreadsheet functionality with XLfit,
and automate with Excel also allows XLfit to update changes. Uniquely
among the packages discussed here, XLfit uses a clever statistical
technique to detect and removeerroneous data points that lie outside
the pattern of the rest of the data. This ‘automatic outlier
rejection’ greatly improves the accuracy of the fit.
Using
Excel data in Prism (www.graphpad.com) is slightly more complicated.
You can’t edit the data within Prism, but you can copy and
paste the relevant portions of your spreadsheet, or paste a link
to the original Excel file, or embed the Excel worksheet. Prism
is a powerful curve-fitting program, as yet little known in this
country. It uses a similar curve-fitting algorithm to XLfit, pick
appropriate statistical tests and interpret the results to make
decisions based on their data. It’s sold directly from the
makers’ Web
site and there doesn’t appear to be
a supplier or support centre in the UK, so you’d
have to be prepared to communicate with California and cope with
the 8-hour time difference when you need technical support.
Another curve-fitting package that isn’t
very well known this side of the Atlantic is DataFit (www.curvefitting.com).
It handles basic non-linear regression, but offers very little
by way of statistics. There is no direct link to Excel, but it
does offer a multitude of ODBC connections to different data sources.
Alternatively, if you have some moderate programming skills, you
could consider DataFitX, which is a COM component for curve-fitting
(an in-process ActiveX DLL) which you include in an Excel macro;
however this is a much moreexpensive solution than DataFit itself.
You can buy XLfit, Prism
or DataFit for roughly half the price of Origin or supported by
Adept Scientific of Letchworth, Herts. As it’s
also the only curve-fitting program that integrates fully with
Excel, it’s
likely to be your first port of call.
Dr Know's recommended download is the XLfit trial version - download yours today.