Using a microcomputer to convert percent response values to probits.

Certain relationships between dose/concentration and percent response/binding in various biochemical systems are sigmoidal rather than linear. This sigmoid relationship is considered linear only over the 20-80 percent range. Appropriate analysis of such data over nearly the entire response/binding range requires the conversion of the percent response data to probits or probability units, producing a linear relationship. While tables of probits are available, the author has incorporated the probability function for probit conversion into a commercially available spreadsheet program. Also included is the Rohlf and Sokal approximation for the area under the normal distribution curve (conversion of probits to cumulative percent). These functions allow use of a spreadsheet to automate the creation of dose-response curves, or standard curves as part of chemical assays (i.e., radioimmunoassay).