Enter the data with all the replicates as shown below. The top part of the table is the standard curve. Below that are the unknown values. The standards and the unknowns do not need to have the same number of replicate determinations.

When you fit the standard curve, select the standard curve option (X from unpaired Y). If you fit the curve with nonlinear regression, this is on the Output options dialog. The example was fit using nonlinear regression with a sigmoidal dose-response curve with variable slope.

The standard curve results are shown on two output views.

The Y column in "standard curve X from Y"  shows the average of the replicate unknown Y values you entered. The X values are the concentrations that correspond to the mean Y values.

Each value in "unknown X values" is a concentration corresponding to one of the replicate values you entered, and is expressed in the same units as the X axis of your standard curve. Because Prism cannot deal with replicate X values, Prism places these unknown X values in a Y column on the results table. Think of them as X values on your standard curve. But think of them as Y values when you want to do further analyses (such as a transform).

To calculate the mean and SD (or SEM) of the replicate values, press Analyze and choose row statistics.