Plotting a Graph

Graphs can be plotted using the Excel Spreadsheet package on the computers in the laboratory. When plotting your graphs, remember:

• Label the axes in the same way as table columns.
• Display error bars.
• The data should fill the whole page, and not just one corner.
• Try to manipulate formulae to give a straight-line relationship.
• A fitted straight line should fit all experimental points to within their errors.

Least Squares Fitting

The analysis of experimental results often involves the drawing of a straight line through the data points. The Linear Regression (or Least Squares) technique allows for the calculation of a such a line, and this procedure is available in Excel (as a Data Analysis option under Tools). In performing a Least Squares calculation, we are making a number of assumptions:

• A linear relationship relationship exists between our two sets of data.
• All points are equally precise.
• The y-data contains all the error.
• There are no obvious outliers in the data set.

Note: If you believe, by inspection of your graph, that one point is an outlier, then it should be excluded from the regression calculation, and a note made to that effect in your write-up.