Replicates and error bars on XY graphs

This page explains how to enter and plot replicates or error bars on XY plots. Read elsewhere about entering and plotting replicates or error bars on Column graphs, Grouped graphs or Survival graphs.

When you create an XY table, choices on the Welcome or New Table dialog let you specify side-by-side subcolumns for entry of replicates or error values computed elsewhere. Learn more about XY tables.

The example below is set for entry of two replicate values (duplicates) but you can choose any number from 2 to 52.

"%CV" is the coefficient of variation, which is equals 100SD/Mean. If you enter the %CV, Prism will plot SD error bars.

If you enter N along with the SD, SEM or %CV, choose to plot the SD, SEM, or 95% CI (via choices on the Format Graph dialog). If you omit N, Prism can only plot the error value you entered.

Note that with XY and Grouped tables, replicates are entered into side-by-side subcolumns. With Column data, replicates are stacked into columns.

Click the Table Format button in the upper left corner of the table to reformat a data table - change the number of replicates, or change the entry of error values.

Use the Format Data Table dialog to specify how you will enter the replicates or error values. If you change from entry of mean and SD to entry of mean and SEM, only the labels change (not the numbers). This lets you correct a mistake (if you were actually entering SEM values but mistakenly set up the table with a subcolumn labeled for SD values). Don't make changes in the Format Data Table dialog when you want to change the way the error bars are plotted. Read on to see how to change error bar plotting.

When you create a new table and enter replicates, you choose not only how the subcolumns are formatted, but also how Prism plots them. You can choose to plot individual replicates, mean only, mean with error bar (which you can specify). Choose from a pair of drop down lists directly below the place where you choose the number of replicates.

If you choose to enter error values directly, Prism will plot the values you entered (except that when you enter %CV, Prism plots the SD). If you entered Mean, SD (or SEM) and N, you can choose (on the graph) to plot the error bars as SD, SEM or 95% CI.

To plot the replicates or error bars differently, you don't have to remake a graph. Instead, click the Change graph type button.

The changes you make on the Change Type of Graph dialog apply to all data sets on the graph.

An alternative approach is to double-click anywhere on the graph to bring up the Format Graph dialog. Then click the "All' button, so changes you make apply to all data sets. If you don't do this, you might accidentally end up with some data sets with one kind of error bar and other data sets with a different kind of error bar.