Many of the projects we end up working on are replacing existing systems with existing data either wholly or in part. In all of the above projects we end up writing data migration or data conversion code to move the data from legacy systems to the new systems. Many stake holders of the project such as business users, project managers, business analysts really care about the data conversion scripts and the quality of the conversion. Since this conversion is business entity related and matters a lot as future business/functionality depends on the data being logically equivalent to the legacy system.
Over the years we have found some techniques that help in testing the quality of the data conversion.