Herman Rubin has five commandments that he posted on UseNet from time to time:
For the client:
1. Thou shalt know that thou must make assumptions.
2. Thou shalt not believe thy assumptions.
For the consultant:
3. Thou shalt not make thy clientâ€™s assumptions for him.
4. Thou shalt inform thy client of the consequences of his assumptions.
For the person who is both (e. g., a biostatistician or psychometrician):
5. Thou shalt keep thy roles distinct, lest thou violate some of the other commandments.
From the comments over at Andrew Gelman’s Blog.