I plotted some data on a log/log graph, and I want to fit a straight line to it. Why does Prism say that the regression will not be on the log values?

The regression analysis fits the data, not the graph. Changing the graph to a log scale doesn't change the fact that the data do not form a straight line. You have three options:

• See this reference on using nonlinear regression to fit a straight line to your data. Then on your log axes this curve will become straight.

• Use the "Transform" analysis to transform your data to their corresponding logs, making a new results table. Be sure to check the option in the transform dialog to make a graph of those new data. Now you are plotting linear data, so linear regression will work.

• Plot the data on a regular axis, so the data are curved, and then fit a curve to that data. Fitting a curve is really not harder than fitting a straight line.