@misc{indiciaeee75da7d32e3,
  title = {Ways of learning: Observational studies versus experiments},
  author = {T.L. Shaffer and Douglas H. Johnson},
  year = {2010},
  doi = {10.2193/2007-293},
  url = {https://pubs.usgs.gov/publication/70033594},
  note = {Source identifier: 70033594}
}