@misc{indiciaea2221d17cf2e,
  title = {Directionality, timing, and sample selection in epidemiologic research design.},
  author = {M S Kramer and J F Boivin},
  year = {1989},
  doi = {10.1016/0895-4356(89)90094-2},
  url = {https://pubmed.ncbi.nlm.nih.gov/2778463/},
  note = {Source identifier: 2778463}
}