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Origin is an easy-to-use data analysis and graphics program that runs in Windows on PCs. These instructions apply to version 3.5 that is available on the HMC server.

- Entering Data
- Plotting Data
- Fixing the Axes
- Fixing the Plot Symbols
- Annotating the Graph
- Fitting Data
- Printing Graphs

Type your data in columns noting the following points:

- Return advances the cursor down the column
- Tab advances the cursor to the right
- Double click on the column name to edit column headings

Use the **File|Import Ascii** command to access the text file. Many
different file configurations can be handled by setting appropriate file import
options.

Select the column holding the **x** data and select **Column|Set as
X**.

If you have a column of errors, select it and issue the command **Column|Set
as Error Bars**

To plot data with error bars, select the *x*, *y*, and errors columns
and issue the command **Plot|Scatter**. It may be useful at this point to
expand the window by clicking the up arrow at the right of the title bar.

By default, Origin adjusts the range of each axis automatically. You can override its choices by double-clicking on the axis and changing settings in the dialog.

Similarly, double click on axis labels to edit them. Buttons are provided in the edit text dialog to allow subscripts, superscripts, Greek characters, boldface, and italics.

By default, Origin does not put a frame around the plot area. You should. Here's how:

**Plot | Show Frame**- Double click on the
*x*axis and click the following check boxes**Axis Top****Top Axis In (Major)**- (
*If you wish*)**Top Axis In (Minor)**

- Then click the
**Goto Y**button and fix the*y*axis in the same way.

Data points should be plotted as individual points with a symbol size that makes sense
for the number of data points in the plot and the plot size. There should **not** be
a line connecting successive points. Points should be shown
with error bars, if available. Make a column of error bars on your data sheet.

- Turn off a line connecting the points by double clicking on the line/plot symbol for the data series of choice in the plot legend
- If you need to plot both data series (in symbols) and smooth curves (in
lines without symbols)

- Click the
**Independent Series**button - Close the dialog
- Double click on the symbol of the series whose symbol/line you wish to modify in the legend

- Click the
- Set the line style and plot symbol for that series
- Repeat for other series as necessary

For a quick and dirty fit, select **Fit|Line** when a plot is front-most.
**Note:** this fit does not pay attention to error bars. It is therefore
useful only when each point has the same error in *y* *and* you don't
care to inquire how confident you should be in the fit. In most cases, you will
want to know the value of *c*^{2} for your fit. To get this, you must
define a fit function, as described in the following section.

As an example, consider a linear fit function of the form *y* = *mx*+ *b*. This function has two free parameters, namely *m* and
*b*. First define the function with **Fit|Select Fitting Function**,
click on **Define New Function**, and enter a name in the **Function
Name** box.

Under **Parameters**, click the **User-defined Names** button, then click
in the **Parameter Names** box and type `m,b`.

Click in the large equation box at the bottom and type the equation
`m*x+b`, then click **OK**.

**Fit|StarFitting Session** starts the fit. Set initial values for each
parameter, then click
**Update Fit Curve** to see the curve plotted on the graph. For a linear
fit, it doesn't matter how close the curve is to the data, **but for a
general nonlinear fit, it is crucial that the initial values of the
parameters be close to the best-fit values**. Adjust the parameters
until the curve "has the spirit of the data", then click
**Fit**. When the fit is done, click **Plot**, then
**OK** to paste the results onto the graph. **Note** the value of Chi^2
is really *c*^{2} per degree of freedom.

From the fitting window click to add the fit information to the plot. A
shadow box with the best values of the fit parameters, their estimated
uncertainties, and Chi^2 is displayed. **The value is really the
reduced** *c*^{2}, that is, *c*^{2} divided by the number of degrees of
freedom.

For including a graph in a technical report or publication, you should copy down the information from the fit, remove it from the graph, and place it in the figure caption.

For use in a lab notebook, it is very convenient to print a version of your graph that is small enough to permit you to annotate the graph and explain its significance on the same notebook page. A graph with a plot area of about 4 inches by 3 inches is quite good for this.

Left to its own devices, Origin will fill the entire page. This is usually bigger than you want. To shrink it down, click on the lower right corner of the plot area until you get a square drag handle. Resize the plot area until it is the size you want. Better yet, double click on the gray box with the number "1" in it at the top-left corner of the plot. Set the units to "inch" and type in 4 inches wide by 3 inches high for the plot area.

Copyright © 2001 Harvey Mudd College Physics Department http://www.physics.hmc.edu/ WebMaster@Physics.hmc.edu This page was last modified on Wed, Jan 21, 1998. |