Error Bars

Error bars allow you to graphically illustrate actual errors, the statistical probability of errors, or a general approximation or "spread" in your data. Examples might include experimental errors in measurement or atypical data points in comparison to the rest of the data.

Error bars appear as 2 short dashes for the upper and lower values of the spread, with a line connecting the 2 dashes at their centres. These 3 parts of the error bar are treated as 1 chart element in the chart, so you can, for example, make an all-red error bar, but you cannot make red dashes with a blue connecting line.You can choose error bar options for all data series, or for an individual data series with different parameters for the upper and lower error bars.

Error bars can be added to the following 2-D charts:-

 

Adding Error Bars to a Chart

In order to fix ideas, let us add error bars to the line graph Tut_Line#1 created earlier in the tutorial. Open this graph if it isn't already open, and select the Chart view.

Now, select Error Bars from the Chart menu. The following dialog box appears:-

In the Series section, Default selects all data series, One Series allows you to choose a particular individual data series. Choose One Series and select S.

Orientation (which applies to Scatter, XY Column, and XY Line charts only) allows each X component and Y component to have independent error bars of any type, including "None".

Direction (which has 3 options: Both, Plus, Minus) allows you to have different error bar parameters for the Plus or Minus bar. For example, you might use the standard error for the Plus, but use a percentage for the Minus. If you want the Plus and Minus bars to be treated identically, select Both.

Options box allows you to choose the type of error bars to be drawn, as follows:-

Cap Length sets the length of the end caps ("whiskers") on the error bars. Default is 0.1 inches, but can be adjusted and set to centimetres, picas, points, or ciceros. This affects all error bars in the selected series.

Clear One Series (or Clear All Series if all series selected) allows you to change the settings of the selected series back to the default setting.

Show allows you to preview your changes without exiting from the dialog box.

(Before trying any of the exercises below, remove the symbol displayed at each data point by double-clicking any symbol and selecting No Symbol in the displayed dialog box).

Exercise-1 (Standard Error)

Select the Standard Error option for the S data series and redraw the chart. It should look like this:-

 

Exercise-2 (%)

Select 20 for the % option for the S data series and redraw the chart. It should look like this:-

 

 

Exercise-3 (Variable)

Select Data View from the View menu, and select the Tut_Line#1 data page, as follows:-

Add error data relating to the S-series in column C as follows:-

Now carry out the following steps:-

An XY Line graph should be plotted as follows:-

Now select Error Bars from the Chart menu and, for the S data series, choose Variable and the Error-S data series, as follows:-

Now click on OK and the graph should be redrawn as follows:-

Save the chart at this point.


 

 

Graphics     Multimedia      Virtual Environments      Visualisation      Contents