Engineered viral and non-viral vectors can be designed as directly administered products or as critical reagents in the manufacture of gene modified cell therapies. These vectors require some form of potency testing prior to use in pre-clinical and clinical development. In vitro cell based assays are commonly used as is the incorporation of more than one read out following a transduction or transfection event. A composite case study will be used to highlight a few questions that are frequently encountered during the design, development, and validation of these types of potency assays and how they were addressed for a dual readout test using a six-point dilution curve at both the level of transduction and readout. Specifically, issues of dilution spacing, data transformation, model selection and goodness of fit assessments will be illustrated.