Population Sampling Methods

Yesenia Villar-Villalobos

IS 280

11/12/09

Prof. Richardson

Defining a Population

“A population must be specific enough to provide readers a clear understanding of the applicability of your study to their particular situation and their understanding of that same population” (Dale, 2006)

How important is validity and reliability?

Type of Sampling Methods

Probability Nonprobability

Probability Sampling

“Gives all members of the population a known chance of being selected for inclusion in the sample and this does not depend upon previous events in the selection process” (Birchall, 2009)

Probability Sampling:– Simple Random– Systematic – Stratified– Cluster

Simple Random Sampling

Define the population Determine percentage to

be interviewed or studied

Each individual has an equal chance of selection

Random sample becomes representative of the larger whole

Systematic Sampling

Every nth element from a list (sample interval)

List must be in random order

Stratified Sampling

Classify the population in sub-populations (strata) based on well-known characteristics

– Ex: Age, gender, socio-economic status, etc.

Randomly chose a sample from sub-populations

Either proportional or disproportional

Cluster Sampling

Randomly select groups (strata) rather than individuals

Multi-stage sampling Useful when no

elements within a population is available

Final sample may not be representative of the population

Nonprobability Sampling

“Members are selected from the population in some nonrandom manner” (Barreiro, 2009)

● Nonprobabilty Sampling: – Convenience – Purposive – Quota

Convenience Sampling

Exploratory research Inexpensive

approximation– Ex: preliminary research

efforts to attain a gross estimate

Saves time and money

Purposive Sampling

“The person who is selecting the sample is who tries to make the sample representative, depending on his opinion or purpose, thus being the representation subject” (Barreiro, 2009)

Subjective judgment (Judgment Sampling)

Requires researcher confidence that the sample truly represents an entire population

Quota Sampling

Nonprobability equivalent of stratified sampling

Convenience or judgment sampling to fill quota from specific sub-groups of a population

– Ex: Interviewer is instructed to interview 50 males between the ages of 18-25

Useful when:– Time is limited– Money restraints– Detailed accuracy is not important

Calculating a Sample Size

Rule of thumb: “the larger the sample size, the more closely your sample data will match that from the population” (Birchall, 2009)

Key factors to consider:– How accurate you wish to be– How confident you are in the results– What budget you have available

Sample Size Calculators

http://www.surveysystem.com/sscalc.htm

http://www.ezsurvey.com/samplesize.html

http://www.macorr.com/ss_calculator.htm

References

Barriero, P.L. and Albandoz, J.P, “Population and sample. Sampling techniques,” http://optimierung.mathematik.uni-kl.de/mamaeusch/veroeffentlichungen/ver_texte/sampling_en.pdf.

Birchall, Joanne, “Sampling and Samples,” http://www.market researchworld.net/index.php?option=com_content&task=view&id=23&Itemid=41.

Dale, Dennis, “Population Sampling Methods for Research Studies: Definitions and Uses,” http://www.associatedcontent.com/article/28368/population_sampling_methods_for_research.html.

StatPac Inc.,“Sampling Methods,” http://www.statpac.com/surveys/sampling.htm.