TY  - RPRT
TI  - Observational studies.
AU  - J F Peipert
AU  - M G Phipps
PY  - 1998
DO  - 10.1097/00003081-199806000-00004
UR  - https://pubmed.ncbi.nlm.nih.gov/9646956/
ID  - 9646956
ER  -