The sigmoidal shape of the standard curve in competitive assays is most accurately fit by a four-parameter logistic model (4PLC). Alternate models such as linear, exponential, or log-log, give inaccurate readings, particularly at high and low concentrations. Most plate readers have the ability to fit data using 4PLC methods from the standard curve. Check with your plate reader’s manufacturer if you are unsure. Alternatively, free data analysis software for all of Arbor Assays’ kits can be found online at