# Sampling.

## Presentation on theme: "Sampling."— Presentation transcript:

Sampling

Population Well-defined set with specified properties People Animals
Events Sport teams Clinical units Communities Schools Specimens Charts Historical documents

Census Investigation of all individual elements that make up a population Sample

Why sample? Generally difficult to study entire population
(Cost + Speed) Able to make generalizations to population from appropriately derived sample.

Sampling Procedure by which some members of the population are selected as representatives of the entire population

Sample Subset of a larger population Population Sample

Sampling Frame

Who do you want to generalize to? The Theoretical Population

Types of samples Nonprobabilistic Probabilistic Nonrandom selection
Can not assure every element has an equal chance for being included Probabilistic Uses some form of random selection More likely to result in representative sample

Probability Sampling Simple random sampling Stratified random sampling
Systematic Sampling Cluster sampling Need a sampling frame Every element has an equal chance of being in the sample

Simple Random Sampling
the purest form of probability sampling. Assures each element in the population has an equal chance of being included in the sample Random number generators

Simple Random Sampling
List of Residents

Simple Random Sampling
List of Residents Random Subsample

STATISTICAL TABLES: Table A Random Digits

Example: Simple random sampling
1 Albert D. 2 Richard D. 3 Belle H. 4 Raymond L. 5 Stéphane B. 6 Albert T. 7 Jean William V. 8 André D. 9 Denis C. 10 Anthony Q. 11 James B. 12 Denis G. 13 Amanda L. 14 Jennifer L. 15 Philippe K. 16 Eve F. 17 Priscilla O. 18 Thomas G. 19 Brian F. 20 Hellène H. 21 Isabelle R. 22 Jean T. 23 Samanta D. 24 Berthe L. 25 Monique Q. 26 Régine D. 27 Lucille L. 28 Jérémy W. 29 Gilles D. 30 Renaud S. 31 Pierre K. 32 Mike R. 33 Marie M. 34 Gaétan Z. 35 Fidèle D. 36 Maria P. 37 Anne-Marie G. 38 Michel K. 39 Gaston C. 40 Alain M. 41 Olivier P. 42 Geneviève M. 43 Berthe D. 44 Jean Pierre P. 45 Jacques B. 46 François P. 47 Dominique M. 48 Antoine C.

SIMPLE RANDOM SAMPLING

Systematic Random Sampling
N = 100 want n = 20 N/n = 5 select a random number from 1-5: chose 4

Systematic Random Sampling
N = 100 want n = 20 N/n = 5 select a random number from 1-5: chose 4 start with #4 and take every 5th unit

Stratified Random Sampling
List of Residents

Stratified Random Sampling
List of Residents surgical medical Non-clinical Strata

Stratified Random Sampling
List of Residents surgical medical Non-clinical Strata Random Subsamples of n/N

Cluster Sampling The primary sampling unit is not the individual element, but a large cluster of elements. Either the cluster is randomly selected or the elements within are randomly selected Why? Frequently used when no list of population available or because of cost

Example: Cluster sampling
Section 1 Section 2 Section 3 Section 5 Section 4

Non-Probability Sampling
Convenience Quota Purposive Snowball Every element does not have an equal chance of being in the sample

Convenience Available subjects enter study until sample size reached
Inexpensive, quick, easy Large risk of bias Questionable representativeness Examples: First 30 patients who enter a clinic with arthritis Parents of children in a shopping mall

Purposive sampling Handpick cases
Conscious effort to include specific elements in sample May pick subjects with diverse views, specific characteristics Easy, bias present, limited representativeness Used in qualitative research

Purposive Sample Examples:
Specific populations: Victims of child abuse Parents of children with rare illness Diverse views: Those who support/don’t support a public policy (e.g., abortion)

Quota Sampling Include specific number of elements in pre-determined categories Based on known pop. characteristics Relatively easy Bias present

Quota Sampling - Example
Quota Sample

Snowball Sampling Networking sampling (snowballing)
Ask for referrals from identified case

Classification of Sampling Methods
Probability Samples Non- probability Systematic Stratified Convenience Snowball Cluster Simple Random Purposive Quota

Sample Size Should be determined by researcher before quantitative study is conducted Use the largest sample possible