TY  - RPRT
TI  - Sampling and external validity.
AU  - G A Morgan
AU  - R J Harmon
PY  - 1999
DO  - 10.1097/00004583-199908000-00023
UR  - https://pubmed.ncbi.nlm.nih.gov/10434499/
ID  - 10434499
ER  -