TY  - RPRT
TI  - Design versus directionality.
AU  - S Greenland
AU  - H Morgenstern
PY  - 1991
DO  - 10.1016/0895-4356(91)90269-f
UR  - https://pubmed.ncbi.nlm.nih.gov/1995777/
ID  - 1995777
ER  -