The text box at the top of the tab allows the user to enter an expression. Double clicking on a metric in the "Performance Data" tree will copy that metrics name into the box. If a metric contains any operands, the whole metric must be surrounded by quotes. If the you would like of the metric to be renamed, then you should start the expression with the new name and and equals sign.

If this is the only metric you wish to derive, then select the trial, expression or application where the metric should be derived and then click apply. If you wish to derive many metrics, then click Add to List and create more expressions.

If you have added multiple expressions, you can select one or many of them to apply. They will be derived from top to bottom. After you have select some, you can select the trial, experiment or application to apply the expression to and then click apply.

You can also derive metrics using an expression file. An expression file has a single expression per line. To parse the file, select the trial, experiment or application to apply the expressions to; then select File > Parse Expression File and chose the file.