Probability sampling (random sampling)

ο  It is a selection process that ensures each participant the same probability of being selected.
ο  Random sampling is the best method for ensuring that a sample is representative of the larger population.
ο  Random sampling can be:

• simple random sampling
• stratified random sampling, and
• cluster sampling.

Nonprobability sampling

ο  It is the selection process in which the probability that any one individual or subject selected is not equal to the probability that another individual or subject may be chosen.
ο  The probability of inclusion and the degree to which the sample represents the population are unknown.
ο  The major problem with nonprobability sampling is that sampling bias can occur.
ο  Nonprobability sampling can be:

• convenience sampling
• purposive sampling or
• quota sampling

nursingplant.com

• An instrument in a research study is a device used to measure the concept of interest in a research project.
• It is used to measure a concept of interest.
• An ideal measuring instrument is one which results in measures that are relevant, accurate, objective, sensitive and efficient.
• Measures which are Physical and physiological have higher chance of success in attaining these goals than measures that are psychological and behavioral.
• Instruments can be:
  • observation scales,
  • questionnaires or
  • interview schedules.
• Validity and reliability are two statistical properties used to evaluate the quality of research instruments (Anastasi, 1986).
• It is important that assessment techniques possess both validity and reliability.

Suresh KP and Chandrashekara, S. Sample size estimation and power analysis for clinical research studies.Journal of Human Reprouductive Sciences. 2012 Jan;5(1):7-13. doi: 10.4103/0974-1208.97779.

While "population" is about the generality of the universe or groups of people, "sample" refers to a fraction of persons from the population.

It is from the sample that data is collected in the course of a study, and because that sample is representative of the population, the results can be generalized to the population in a scientific study.