Example: Interpolating from a sigmoidal standard curve

1. Create the data table

From the Welcome or New Table dialog, choose to create an XY data table, and select the sample data "Interpolate unknowns from standard curve (RIA)".

2. Inspect the data

The sample data may be partly covered by a floating note explaining how to fit the data (for people who are not reading this help page). You can move the floating note out of the way, or minimize it.

The first seven rows contain the standard curve, in duplicate. Below that are three unknown values. These have a Y values that you measured, but no X. The goal of this analysis is to interpolate the corresponding X values (concentrations) for these unknowns.  Note that three of the four unknowns are labeled, so you can later match up the results with the labels.

3. View the graph

The graph Prism makes automatically is fairly complete. You can customize the symbols, colors, axis labels, etc. Since the unknowns have no X value, they are not included on the graph.

4. Choose nonlinear regression

Click and choose Nonlinear regression from the list of XY analyses.

Alternatively, click the shortcut button for nonlinear regression.

5. Choose a model

On the Fit tab of the nonlinear regression dialog, open the panel of inhibitory dose-response models and choose: log(inhibitor) vs. response -- variable slope.

At the bottom of the dialog, check the option: Interpolate unknowns from standard curve.

For this example, leave all the other settings to their default values.

Click OK to see the curves superimposed on the graph.

6. Inspect the results

The results appear on several pages. The first page is the table of results. It tabulates the best-fit values of the parameters and much more. For this example, we aren't too interested in these results. The next two pages show the results we care about here -- the interpolated X values

Since the data table was formatted for entry of duplicate Y values, Prism presents the interpolated values on two pages. One page averages the duplicates and interpolates the average. The other page reports the interpolation of each entered Y value. For this example, we only entered unknown values into one subcolumn so the two interpolated results pages are identical.

7. Transform the results

The X column of the results table has the interpolated values we want. These are in the same units as the X values, so are the logarithm of concentration. Prism can transform these values to concentration units.

Click and choose Transform at the top of the Analyze dialog.

On the Transform dialog check the option to transform X values and choose the transform X=10^X

Now the X column is in molar concentration units. Note that the column title hasn't changed. Prism isn't smart enough to adjust the column titles when you transform data.

Click and edit the column title to "Concentration (M)".