Intervention fidelity: lessons learned from an unsuccessful pilot study.

Although the design of the study we undertook was randomized, the groups were statistically similar, the intervention was based on evidence of previous efficacy, and the treatment was delivered exactly as planned, this pilot study on an intervention to promote motivation for weight loss in new mothers failed to produce an effect. A closer examination using criteria for intervention fidelity revealed a number of weaknesses. Both treatment integrity and differentiation merit careful consideration in intervention design.