TY  - RPRT
TI  - Directionality, timing, and sample selection in epidemiologic research design.
AU  - M S Kramer
AU  - J F Boivin
PY  - 1989
DO  - 10.1016/0895-4356(89)90094-2
UR  - https://pubmed.ncbi.nlm.nih.gov/2778463/
ID  - 2778463
ER  -