18. References:
1. Lance, P.; Hattori, A. (2016). Sampling and Evaluation.
Web: MEASURE Evaluation. pp. 6–8, 62–64.
2. Salant, Priscilla, I. Dillman, and A. Don. How to conduct
your own survey. No. 300.723 S3. 1994.
3. Robert M. Groves; et al. (2009). Survey methodology.
ISBN 978-0470465462.
4. Lohr, Sharon L. Sampling: Design and analysis.