@misc{indiciaedd0ddd07cf87,
  title = {Sampling and external validity.},
  author = {G A Morgan and R J Harmon},
  year = {1999},
  doi = {10.1097/00004583-199908000-00023},
  url = {https://pubmed.ncbi.nlm.nih.gov/10434499/},
  note = {Source identifier: 10434499}
}