MS-EXCEL: GOING FURTHER WITH GRAPHS I: ADDING ERROR BARS

Error bars are way of showing the degree of uncertainty in a set of data. MS Excel can add them to your graphs.

Error Bars Showing Precision

The data below show the results from an experiment where the seeds from three species of plants were germinated for one week. At the end of the week the numbers of seeds germinating were counted. 25 seeds were used in each trial.

First the data is entered onto a spread sheet. The spread sheet is used to calculate the mean and the mean percentage germination. Then a bar chart is drawn using the graphing menu.

To enter the error bars first left click on one of the columns. This activates all the columns.

Double left click and a menu will appear. Select error bars (Fr Barre d’erreur). As this is a bar chart it will only offer to draw error bars on the y-axis.

Select precision and enter 4. The data is calculated from samples of 25 seeds so the degree of precision is ± 1 seed. This means each seed represents 1/25 or 4% uncertainty.

 Press Enter or click OK and you will see the error bars appear on the graph.

Note: You will notice that you also have the possibility to enter percentage error, it should not be used this time as it will calculate 4% of each data point so the error bars will be proportional to the data (try it and you will see).

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© Paul Billiet 2010