Introduction Definition Types of Sampling Methods
Population and SampleECON408 (Reserach Methods in Economics)
Pairach Piboonrugnroj, PhD
Faculty of Economics, Chiang Mai Universityme (at) pairach (dot) com
2016
This course is a part of Bachelor of Economics at Chiang Mai University, Thailand
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Outline
What we will learn in this topic
1 Introduction2 Definition3 Types of Sampling Methods
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Introduction
what is the population and sample in research?
Write down your definition of populationand sample on a paper (2 minutes)
Discusswith a person next to you. Compare andcontrast your definitions (5 minutes)
Revise your definition if any
Share with the class
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Definition of Population
Population is a complete set of elements (persons or objects)that possess some common characteristic defined by thesampling criteria established by the researcher
Composed of two groups - target population & accessiblepopulation
Target population (universe) is the entire group of people orobjects to which the researcher wishes to generalize the studyfindings. It meet set of criteria of interest to researcher
Accessible population is the portion of the population to whichthe researcher has reasonable access; may be a subset of thetarget population. May be limited to region, state, city, county,or institution
source: http://www.umsl.edu/ lindquists/sample.html
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Definition of Samples
Terminology used to describe samples and sampling methodsare:
Sample = the selected elements (people or objects) chosen forparticipation in a study; people are referred to as subjects orparticipants
Sampling = the process of selecting a group of people, events,behaviors, or other elements with which to conduct a study
Sampling frame = a list of all the elements in the populationfrom which the sample is drawn
source: http://www.umsl.edu/ lindquists/sample.html
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Sampling frame
Sampling frame could be extremely large if population isnational or international in nature. Frame is needed so thateveryone in the population is identified so they will have anequal opportunity for selection as a subject (element).Examples:
1 A list of all tourism companies that are the member of theChiang Mai Chamber of Commerce
2 A list of Economics students who are the member ofstudent association
3 A list of all children with disability who study in Chiang Mai
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Sampling terminology
1 Randomization = each individual in the population has anequal opportunity to be selected for the sample
2 Representativeness = sample must be as much like thepopulation in as many ways as possible
3 Parameter = a numerical value or measure of acharacteristic of the population; remember P forparameter & population
4 Statistic = numerical value or measure of a characteristicof the sample; remember S for sample & statistic
5 Precision = the accuracy with which the populationparameters have been estimated; remember thatpopulation parameters often are based on the samplestatistics
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Types of Sampling Methods
There are two main types of sampling methods:probability and non-probability
Either of both method shall be selected carefully
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Probability Sampling Methods
Also called random sampling
Every element (member) of the population has aprobability greater than) of being selected for the sample
Everyone in the population has equal opportunity forselection as a subject
Increases sample’s representativeness of the population
Decreases sampling error and sampling bias
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Types of probability sampling
Simple random
Stratified random
Cluster random sampling
Systematic
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Types of probability sampling
Elements selected at random
Assign each element a number
Select elements for study by1 using a table of random numbers in book2 Computer generated random numbers table3 Draw numbers for box (hat)
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Stratified random
Population is divided into subgroups, called strata, accordingto some variable or variables in importance to the study
Variables often used include: age, gender, ethnic origin,SES, diagnosis, geographic region, institution, or type ofcareTwo approaches to stratification - proportional &disproportional
1 Proportional = Subgroup sample sizes equal theproportions of the subgroup in the population
2 Disproportional = Subgroup sample sizes are not equal tothe proportion of the subgroup in the population
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Cluster random sampling
A random sampling process that involves stages of sampling.The population is first listed by clusters or categories. Theprocedure are:
Randomly select 1 or more clusters and take all of theirelements (single stage cluster sampling); e.g. Northernregion of Thailand
Or, in a second stage randomly select clusters from thefirst stage of clusters; eg 3 provinces in Northern region ofThailand
In a third stage, randomly select elements from thesecond stage of clusters; e.g. 30 county health dept.nursing administrators from each state
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Systematic
A random sampling process in which every kth (e.g. every 5thelement) or member of the population is selected for thesample after a random start is determined. Example:
Population (N) = 2000, sample size (n) = 50, k=N/n, so k = 2000 ) 50 =40
Use a table of random numbers to determine the starting point forselecting every 40th subject
With list of the 2000 subjects in the sampling frame, go to the startingpoint, and select every 40th name on the list until the sample size isreached. Probably will have to return to the beginning of the list tocomplete the selection of the sample.
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Non-probability sampling methods
Issues to consider for Non-probability sampling methods.
Characteristics
Types of non-probability sampling methods
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Characteristics
Not every element of the population has the opportunityfor selection in the sample
No sampling frame
Population parameters may be unknown
Non-random selection
More likely to produce a biased sample
Restricts generalization
Historically, used in most nursing studies
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Types of non-probability sampling methods
Convenience - aka chunk, accidental & incidentalsampling
Quota
Purposive - aka judgmental or expert’s choice sampling
Snowball
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Convenience - aka chunk, accidental & incidentalsampling
Selection of the most readily available people or objectsfor a study
No way to determine representativeness
Saves time and money
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Quota
Selection of sample to reflect certain characteristics ofthe population
Similar to stratified but does not involve random selection
Quotas for subgroups (proportions) are established
E.g. 50 males & 50 females; recruit the first 50 men andfirst 50 women that meet inclusion criteria
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Purposive - aka judgmental or expert’s choicesampling
Researcher uses personal judgement to select subjectsthat are considered to be representative of the population
Handpicked subjects
Typical subjects experiencing problem being studied
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Snowball
Also known as network sampling
Subjects refer the researcher to others who might berecruited as subjects
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Time Frame for Studying the Sample
See design notes on longitudinal & cross-sectional studies
Longitudinal
Cross-sectional
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Sample Size
General rule - as large as possible to increase therepresentativeness of the sample
Increased size decreases sampling error
Relatively small samples in qualitative, exploratory, casestudies, experimental and quasi-experimental studies
Descriptive studies need large samples; e.g. 10 subjectsfor each item on the questionnaire or interview guide
As the number of variables studied increases, the samplesize also needs to increase in order to detect significantrelationships or differences
A minimum of 30 subjects is needed for use of the centrallimit theorem (statistics based on the mean)
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Large samples are needed if:
There are many uncontrolled variables
Small differences are expected in the sample/populationon variables of interest
The sample is divided into subgroups
Dropout rate (mortality) is expected to be high
Statistical tests used require minimum sample orsubgroup size
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Power Analysis (I)
Power analysis = a procedure for estimating either thelikelihood of committing a Type II error or a procedure forestimating sample size requirements.
Determine the sample size
Background Information for Understanding PowerAnalysis: Type I and Type II errors
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Type I Error
Based on the statistical analysis of data, the researcherwrongly rejects a true null hypothesis; and therefore,accepts a false alternative hypothesisProbability of committing a type I error is controlled by theresearcher with the level of significance, alpha.Alpha a is the probability that a Type I error will occurAlpha a is established by researcher; usually a = .05 or .01Alpha a = .05 means there is a 5% chance of rejecting atrue null hypothesis; OR out of 100 samples, a true nullhypothesis would be rejected 5 times out of 100 andaccepted 95 times out of 100.Alpha a = .01 means there is a 1% chance of rejecting atrue null hypothesis; OR out of 100 samples, a true nullhypothesis would be rejected 1 time out of 100 andaccepted 99 times out of 100
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Type II error
Based on the statistical analysis of data, the researcherwrongly accepts a false null hypothesis; and therefore,rejects a true alternate hypothesisProbability of committing a Type II error is reduced by apower analysis
1 Probability of a Type II error is called beta b2 Power, or 1- b is the probability of rejecting the null
hypothesis and obtaining a statistically significant result
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Sampling Error and Sampling Bias
Sampling error = The difference between the sample statistic(e.g. sample mean) and the population parameter (e.g.population mean) that is due to the random fluctuations indata that occur when the sample is selected.Sampling bias:
Also called systematic bias or systematic variance
The difference between sample data and population datathat can be attributed to faulty sampling of the population
Consequence of selecting subjects whose characteristics(scores) are different in some way from the populationthey are suppose to represent
This usually occurs when randomization is not used
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Randomization Procedures in Research
Randomization = each individual in the population has anequal opportunity to be selected for the sample
Random selection = from all people who meet theinclusion criteria, a sample is randomly chosen
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
Introduction Definition Types of Sampling Methods
Q&A
Pairach Piboonrugnroj, PhD Faculty of Economics, Chiang Mai University
ECON304 - 02. Index number
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