Experimentation in software engineering is difficult. One reason is that there are a large number of context variables, and so creating a cohesive understanding of experimental results requires a mechanism for motivating studies and integrating results. It requires a community of researchers that can replicate studies, vary context variables, and build abstract models that represent the common observations about the discipline. This paper discusses the experience of the authors, based upon a collection of experiments, in terms of a high level framework for organizing sets of related studies. With such a framework, experiments can be viewed as part of common families of studies, rather than being isolated events. Common families of studies can contribute to higher level hypotheses that no individual experiment could achieve. Then the replication of experiments within a family of studies can act as the cornerstone for building knowledge in an incremental manner. A mechanism is suggested that motivates, records, and integrates individual experiments within a family for analysis by the community at large. To support the framework, this paper discusses the experiences of the authors in carrying out empirical studies, with specific emphasis on persistent problems encountered in experimental design, threats to validity, criteria for evaluation, and execution of experiments in the domain of software engineering. (Also cross-referenced as UMIACS-TR-99-05)