populations and samples dr. fred mugambi mwirigi jkuat
TRANSCRIPT
Populations and SamplesPopulations and Samples
Dr. Fred Mugambi MwirigiDr. Fred Mugambi Mwirigi
JKUATJKUAT
IntroductionIntroduction
A sample is a subset of a larger population of A sample is a subset of a larger population of objects individuals, households, businesses, objects individuals, households, businesses, organizations and so forthorganizations and so forth
Sampling enables researchers to make Sampling enables researchers to make estimates of some unknown characteristics of estimates of some unknown characteristics of the population in questionthe population in question
A finite group is called population whereas a A finite group is called population whereas a non-finite (infinite) group is called universenon-finite (infinite) group is called universe
A census is a investigation of all the individual A census is a investigation of all the individual elements of a population elements of a population
Population Vs SamplePopulation Vs Sample
PopulationPopulation
SampleSample
Reasons for SamplingReasons for Sampling Budget and time Constraints (in case of large populations)Budget and time Constraints (in case of large populations)
High degree of accuracy and reliability (if sample is High degree of accuracy and reliability (if sample is representative of population) representative of population)
Sampling may sometimes produce more accurate results Sampling may sometimes produce more accurate results than taking a census as in the latter, there are more risks than taking a census as in the latter, there are more risks for making interviewer and other errors due to the high for making interviewer and other errors due to the high volume of persons contacted and the number of census volume of persons contacted and the number of census takers, some of whom may not be well-trained takers, some of whom may not be well-trained
The Sampling ProcessThe Sampling Process
Define the Targetpopulation
Define the Targetpopulation
Select a Sampling Frame
Select a Sampling Frame
Determine if a probability or non-probability sampling
method will be chosen
Determine if a probability or non-probability sampling
method will be chosen
Plan procedure for selecting sampling units
Plan procedure for selecting sampling units
Determine sample sizeDetermine sample size
Select actual sampling unitsSelect actual sampling units
Conduct fieldworkConduct fieldwork11
22
33
44
55
66
77
Defining the Target PopulationDefining the Target Population The target population is that complete group The target population is that complete group
whose relevant characteristics are to be whose relevant characteristics are to be determined through the samplingdetermined through the sampling
A target population may be, for example, all A target population may be, for example, all faculty members in the School for Human faculty members in the School for Human Resource Development at JKUA, all housewives Resource Development at JKUA, all housewives in Mombasa or even all past students of a in Mombasa or even all past students of a university university
The Sampling FrameThe Sampling Frame The sampling frame is a list of all those population The sampling frame is a list of all those population
elements that will be used in the sampleelements that will be used in the sample
Examples of sampling frames are a student database Examples of sampling frames are a student database (for the student population), the list of companies on the (for the student population), the list of companies on the stock exchange or the yellow pages (for businesses)stock exchange or the yellow pages (for businesses)
Often, the list does not include the entire population. The Often, the list does not include the entire population. The discrepancy is often a source of error associated with the discrepancy is often a source of error associated with the selection of the sample (sampling frame error)selection of the sample (sampling frame error)
Sampling UnitsSampling Units The sampling unit is a single element – or The sampling unit is a single element – or
group of elements – subject to selection in group of elements – subject to selection in a sample. Examples:a sample. Examples: Every student at JKUAT whose first name Every student at JKUAT whose first name
begins with the letter “F”begins with the letter “F” All child passengers under 18 years of age All child passengers under 18 years of age
who are traveling in a bus from destination X who are traveling in a bus from destination X to destination Y to destination Y
Sampling ErrorsSampling ErrorsRandom Sampling ErrorRandom Sampling Error This is defined as the “This is defined as the “difference between the sample difference between the sample
result and the result of a census conducted using result and the result of a census conducted using identical procedures”identical procedures” and is the result of chance and is the result of chance variation in the selection of sampling unitsvariation in the selection of sampling units
If samples are selected properly (for e.g. through the If samples are selected properly (for e.g. through the technique of randomization), the sample is usually technique of randomization), the sample is usually deemed to be a good approximation of the population deemed to be a good approximation of the population and thus capable of delivering an accurate resultand thus capable of delivering an accurate result
Systematic (Non-Sampling) ErrorsSystematic (Non-Sampling) Errors
These errors result from factors such as an These errors result from factors such as an improper improper research designresearch design that causes response error or from that causes response error or from errors committed in the execution of the research, errors errors committed in the execution of the research, errors in recording responses and non-responses from in recording responses and non-responses from individuals who were not contacted or who refused to individuals who were not contacted or who refused to participateparticipate
Both Random sampling errors and systematic (non-Both Random sampling errors and systematic (non-sampling) errors reduce the representativeness of a sampling) errors reduce the representativeness of a sample and consequently the value of the information sample and consequently the value of the information which is derived by business researchers from it which is derived by business researchers from it
Probability and Probability and Non-Probability SamplingNon-Probability Sampling
Probability Sampling – Every element in the Probability Sampling – Every element in the population under study has a non-zero population under study has a non-zero probability of selection to a sample, and every probability of selection to a sample, and every member of the population has an equal member of the population has an equal probability of being selectedprobability of being selected
Non-Probability Sampling – An arbitrary means Non-Probability Sampling – An arbitrary means of selecting sampling units based on subjective of selecting sampling units based on subjective considerations, such as personal judgment or considerations, such as personal judgment or convenience. convenience.
Non-Probability SamplingNon-Probability Sampling
1.1. Convenience SamplingConvenience Sampling
2.2. Judgment (purposive) SamplingJudgment (purposive) Sampling
3.3. Quota SamplingQuota Sampling
4.4. Snowball SamplingSnowball Sampling
Convenience SamplingConvenience Sampling This is a sampling technique which selects those This is a sampling technique which selects those
sampling units most conveniently available at a certain sampling units most conveniently available at a certain point in, or over a period, of time point in, or over a period, of time
Major advantages of convenience sampling is that is quick, Major advantages of convenience sampling is that is quick, convenient and economical; a major disadvantage is that the convenient and economical; a major disadvantage is that the sample may not be representativesample may not be representative
Convenience sampling is best used for the purpose of Convenience sampling is best used for the purpose of exploratory research and supplemented subsequently with exploratory research and supplemented subsequently with probability samplingprobability sampling
Judgment (purposive) SamplingJudgment (purposive) Sampling A sampling technique in which the business A sampling technique in which the business
researcher selects the sample based on researcher selects the sample based on judgment about some appropriate characteristic judgment about some appropriate characteristic of the sample membersof the sample members
Quota Sampling Quota Sampling A sampling technique in which the business researcher A sampling technique in which the business researcher
ensures that certain characteristics of a population are ensures that certain characteristics of a population are represented in the sample to an extent which he or she represented in the sample to an extent which he or she desiresdesires
Example: A business researcher wants to determine through Example: A business researcher wants to determine through interview, the demand for Product X in a district which is very interview, the demand for Product X in a district which is very diverse in terms of its ethnic composition. If the sample size is to diverse in terms of its ethnic composition. If the sample size is to consist of 100 units, the number of individuals from each ethnic consist of 100 units, the number of individuals from each ethnic group interviewed should correspond to the group’s percentage group interviewed should correspond to the group’s percentage composition of the total population of that district composition of the total population of that district
Contd.Contd. Quota Sampling has several advantages and disadvantages:Quota Sampling has several advantages and disadvantages:
Advantages include the speed of data collection, less cost, Advantages include the speed of data collection, less cost,
the element of convenience, and representativeness (if the the element of convenience, and representativeness (if the
subgroups in the sample are selected properly) subgroups in the sample are selected properly)
Disadvantages include the element of subjectivity Disadvantages include the element of subjectivity
(convenience sampling rather than probability-based which (convenience sampling rather than probability-based which
leads to improper selection of sampling units) leads to improper selection of sampling units)
Snowball SamplingSnowball Sampling A sampling technique in which individuals or organizations are A sampling technique in which individuals or organizations are
selected first by probability methods, and then additional selected first by probability methods, and then additional respondents are identified based on information provided by the first respondents are identified based on information provided by the first group of respondentsgroup of respondents
Example: Through a sample of 500 individuals, 20 scuba-diving Example: Through a sample of 500 individuals, 20 scuba-diving enthusiasts are identified which, in turn, identify a number of enthusiasts are identified which, in turn, identify a number of other scuba-diversother scuba-divers
The advantage of snowball sampling is that smaller sample The advantage of snowball sampling is that smaller sample sizes and costs are necessary; a major disadvantage is that the sizes and costs are necessary; a major disadvantage is that the second group of respondents suggested by the first group may second group of respondents suggested by the first group may be very similar and not representative of the population with that be very similar and not representative of the population with that characteristic characteristic
Probability SamplingProbability Sampling
1.1. Simple Random SamplingSimple Random Sampling
2.2. Systematic SamplingSystematic Sampling3.3. Stratified SamplingStratified Sampling
Simple Random SamplingSimple Random Sampling This is a technique which ensures that each This is a technique which ensures that each
element in the population has an equal chance element in the population has an equal chance of being selected for the sampleof being selected for the sample
Example: Choosing raffle tickets from a drum, Example: Choosing raffle tickets from a drum, computer-generated selections, random-digit computer-generated selections, random-digit telephone dialingtelephone dialing
The major advantage of simple random sampling is The major advantage of simple random sampling is its simplicityits simplicity
Systematic SamplingSystematic Sampling This is a technique which in which an initial starting point This is a technique which in which an initial starting point
is selected by a random process, after which every is selected by a random process, after which every nthnth number on the list is selected to constitute part of the number on the list is selected to constitute part of the samplesample
Example: From a list of 1500 name entries, a name on the list is Example: From a list of 1500 name entries, a name on the list is randomly selected and then (say) every 25randomly selected and then (say) every 25 thth name thereafter. name thereafter. The sampling interval in this case would equal 25.The sampling interval in this case would equal 25.
For systematic sampling to work best, the list should be random For systematic sampling to work best, the list should be random in nature and not have some underlying systematic pattern in nature and not have some underlying systematic pattern
Stratified SamplingStratified Sampling A technique which in which simple random sub samples are drawn A technique which in which simple random sub samples are drawn
from within different strata that share some common characteristic from within different strata that share some common characteristic
Example: The student body of JKUAT is divided into two groups Example: The student body of JKUAT is divided into two groups (management science, engineering) and from each group, (management science, engineering) and from each group, students are selected for a sample using simple random sampling students are selected for a sample using simple random sampling in each of the two groups, whereby the size of the sample for in each of the two groups, whereby the size of the sample for each group is determined by that group’s overall strength each group is determined by that group’s overall strength
Stratified Sampling has the advantage of giving more Stratified Sampling has the advantage of giving more representative samples and less random sampling error. representative samples and less random sampling error. However, it is more complex and information on the strata may be However, it is more complex and information on the strata may be difficult to obtaindifficult to obtain
OthersOthers
1.1. Cluster Sampling Cluster Sampling
2.2. Multistage Area Sampling Multistage Area Sampling
3.3. Internet SamplingInternet Sampling
Issues in Sample Design and SelectionIssues in Sample Design and Selection
Accuracy – Samples should be Accuracy – Samples should be representative of the target populationrepresentative of the target population
Resources – Time, money and individual Resources – Time, money and individual or institutional capacity are very important or institutional capacity are very important considerations due to the limitation on considerations due to the limitation on them. Often, these resources must be them. Often, these resources must be “traded” against accuracy“traded” against accuracy
Issues in Sample Design and SelectionIssues in Sample Design and Selection
Availability of Information – Often information on sample Availability of Information – Often information on sample participants in the form of lists, directories etc. is participants in the form of lists, directories etc. is unavailable which makes some sampling techniques unavailable which makes some sampling techniques (e.g. systematic sampling) impossible to undertake(e.g. systematic sampling) impossible to undertake
Geographical Considerations – The number and Geographical Considerations – The number and dispersion of population elements may determine the dispersion of population elements may determine the sampling technique used (e.g. cluster sampling)sampling technique used (e.g. cluster sampling)
End End