A year-long case study of the effects of an educational and vocational rehabilitation program in a youth training center was discussed in terms of fourteen issues which might threaten the validity of an evaluation or research project. This correctional institution housed women aged 14 to 21 who were long-term wards of the state, held on remand, or sentenced. The evaluator, who taught a class within the institution, conducted a formative evaluation using observation, interviews, casual conversations, and written records. The evaluation was concerned with both internal validity (program effectiveness for these women and this institution) and external validity (generalizability regarding the funding of similar programs). Threats to the validity of conclusions made about a program were described: (1) historical events previous to the program which affected results; (2) participants' maturation; (3) effects of repeated testing; (4) changes in tests or evaluator's attitudes; (5) statistical regression; (6) attrition; (7) small sample size; (8) bias in selection of sample; (9) obtrusiveness of the inquiry; (10) participant anxiety; (11) diffusion of different treatments; (12) construct validity of causes and effects; (13) communication in problems; and (14) reliance on hearsay evidence rather than direct observation. (GDC)