We examine business planning longitudinally, using a comprehensive empirical study with a range of performance variables from gestation to return on investment. We ask – “How effective is business planning for entrepreneurial ventures”? Unique to the study of business planning, we examine the impact of business planning change longitudinally. Secondly, we examine the importance of study replication and data extension, analyzing a previously published SMJ paper (Delmar and Shane, 2003) using a much longer time horizon. We show the importance of data replication, extension, and the consequences of sub-set selection of data populations.