Although complete program evaluation includes assessment of the program implementation process as well as examination of impact and outcome variables, such process evaluation does not always occur. During field implementation programs may be changed, or re-invented, by those adopting the innovation. A case study of the Arthritis Self-Care Project, a rural community-based intervention study, demonstrates the importance of process evaluation in determining the actual independent variable. Instances of re-invention uncovered by the Arthritis Self-Care Project are explored, and suggestions are provided for dealing with the re-invention inevitable in field research.