Slope-ratio assays are very similar to parallel-line assays insofar as both are indirect analytical assays, which aim to estimate the potency of a test sample relative to that of a standard. Slope-ratio assays are most often encountered in microbiologic applications. As with a parallel-line assay, the key assumption is that the expected response to a given dose of the test sample is the same as that which would be obtained from the standard sample administered at a dose that is p times the given dose. As in parallel-line assays, the parameter p is known as the relative potency.
Unlike the parallel-line assay, the expected response is linear in the dose, not its logarithm. The graphs of the response against dose for test and standard samples will be straight lines of different slopes and common intercept, as the assumptions of the assay demand the same outcome for standard and test samples when the dose is zero. The analysis can be accomplished by a multiple regression, with the estimator of p being the ratio of the slope estimators. Interval estimates are often obtained using Fieller's theorem.
You can see a sample graph on the left. With PLA 3.0, you are able to calculate slope-ratio assays for each standard/test sample combination in your bioassay seperately or simultaneuosly.