Using APIs is a common and often difficult task for developers. Successful API designs can guide users of an API and reduce their dependence on documentation; however API design is far from a science. Usability lab studies have been shown to be successful at improving the usability of specific APIs; however these are expensive and not always possible to run for every API. This paper describes an approach to generalize from studies of specific APIs to investigate the usability impact of design choices that commonly arise in the creation of APIs. Based on these results we will inform the design of many new APIs. A preliminary usability study of whether or not to require constructor parameters confirms our belief that the answer to common design decisions is not always obvious, and making the wrong choice can have a strong negative impact on usability for large groups of API users.