The design of chemical libraries is usually informed by pre-existing characteristics and desired features. On the other hand, assesing the prospective performance of a new library is more difficult. Importantly, a given screening library is often screened in a variety of systems which can differ in cell lines, readouts, formats and so on. In this study we explore to what extent pre-existing libraries can shed light on the relation between library activity and assay features. Using an ontology such as the BAO, it is possible to construct a hierarchy of annotations associated with an assay. Based on this annotation hierarchy we can then ask how likely are molecules associated with a specific annotation, to be identified as active. To allow generalization we consider substrucural features, as represented by a structural key fingerprint, rather than whole molecules. We employ a Bayesian framework to quantify the the association between a substructural feature and a given assay annotation, using a set of NCGC assays that have been annotated with BAO terms. We discuss our approach to training the Bayesian model and describe benchmarks that characterize model performance relative to the position of the annotation in the BAO hierarchy. Finally we discuss the role of this approach in a library design workflow that includes traditional design features such as chemical space coverage and physicochemical properties but also takes in to account screening platform features.