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 When X values are logarithms

In many situations, you'll fit the standard curve to a log(dose) vs. response curve. In these cases, you'll enter the X values for the standard curve as the logarithm of concentration or you'll transform the data to logarithms before running nonlinear regression.

The interpolated X values are in the same units as the X values the analysis sees. If you are analyzing data entered or transformed so X is the logarithm of concentration, then the interpolated values will also be logarithms.

If you want to transform these results back to concentrations, that is easy to do:

1.From the interpolated results page of results, click Analyze.

2.Choose Transform at the very top of the Analyze dialog.

3.Choose the standard list of transforms.

4.Depending on whether your interpolated values are in the X or Y column, choose to transform that column.

5.From the list of transforms, choose Y=10^Y or X=10^X. These choices are about one third of the way down the long list of transforms.

6.The results sheets will have the interpolated values as concentrations, rather than logs.

Why 10^Y or 10^X? Those are abbreviations for 10Y or 10X , which are the antilogarithm functions. Review logarithms and antilogarithms.