Sampling Design and Sampling Distributions

Presented By:• Mudit Singla (51)• Vikas Sonwane (53)• Manisha Tripathy(55)• Vaibhav Sood (57)• Aayush Velaga (59)

Target Population

The target population is the collection of elements or

objects that possess the information sought by the

researcher and about which inferences are to be made.

Terminology

– An element is the object about which or from which the

information is desired, e.g., the respondent

– A sampling unit is an element, or a unit containing the

element, that is available for selection at some stage of the

sampling process

– Extent refers to the geographical boundaries

– Time is the time period under consideration

Important qualitative factors that determine the sample size

– The importance of the

decision

– The nature of the

research

– The number of variables

– The nature of the

analysis

– Sample sizes used in

similar studies

– Incidence rates

– Completion rates

– Resource constraints

The Sampling

Frame

Define the target population

Select a sampling frame

Conduct fieldwork

Determine if a probability or non-probability sampling method will be chosen

Plan procedure for selecting sampling units

Determine sample size

Select actual sampling units

Statistical Errors

The difference between the value of a sample statistic of interest and the value of the

corresponding population parameter a statistical error has occurred.

Types of Errors

Random Sampling Error• The difference between the

sample result and the result of a census conducted using identical procedures

• These errors are due to chance fluctuations

Systematic Error• Systematic (non sampling)

errors result from non sampling factors, primarily the nature of a study’s design and the correctness of execution

• These are not due to chance fluctuations

Illustration

Classification of Sampling Techniques

Sampling Techniques

NonprobabilitySampling Techniques

ProbabilitySampling Techniques

ConvenienceSampling

JudgmentalSampling

QuotaSampling

SnowballSampling

SystematicSampling

StratifiedSampling

ClusterSampling

Other SamplingTechniques

Simple RandomSampling

Types of Non probability sampling

Convenience Sampling

Judgment Sampling

Quota Sampling

Snowball sampling

• The sampling procedure of obtaining those people or units that are most conveniently available.

• Best used for exploratory research.

Convenience Sampling

• A non probability sampling technique in which an experienced individual selects the sample based on personal judgment about some appropriate characteristics of the sample member

Judgment Sampling

• A non probability sampling procedure that ensures that various subgroups of a population will be represented on pertinent characteristics to the exact extent that the investigator desires.

• POSSIBLE SOURCES OF BIAS– haphazard selection of subjects

• ADVANTAGES– Speed of data collection– Lower costs– Convenience

Quota Sampling

• A sampling procedure in which initial respondents are selected by probability methods and additional respondents are obtained from information provided by the initial respondents.

• It uses referrals for selecting respondents• ADVANTAGES

– Reduced sample size– Reduced cost

Snowball sampling

Probability Sampling

The sampling techniques where selection procedure is based on chance are called

probability sampling techniques.

Types of Probability Sampling

Simple Random Sampling

Systematic Sampling Stratified Sampling

Proportional versus

Disproportional Sampling

Cluster Sampling Multistage area sampling

The sampling procedure that ensures each element in the population will have an equal chance of being included in the sample is called simple random sampling.

Simple Random Sampling

A sampling procedure in which a starting point is selected by a random process and then every nth number on the list is selected.

Systematic Sampling

A probability sampling procedure in which simple random subsamples that are more or less equal on some characteristic are drawn from within each stratum of population.

Stratified Sampling

ProportionalA stratified sample in which the number of sampling units drawn from each stratum is in proportion to the population size of that stratum.

DisproportionalA stratified sample in which the sample size for each stratum is allocated according to analytical considerations

Proportional versus

Disproportional Sampling

An economically efficient sampling technique in which the primary sampling unit is not the individual element in the population but a cluster of element; clusters are selected randomly.

Cluster Sampling

Sampling that involves using a combination of two or more probability sampling techniques

Multistage area sampling

Selecting an Appropriate Sample Design

A researcher who must decide on the most appropriate sample design for a specific project willidentify a number of sampling criteria and evaluate the relative importance of each criterion before selecting a sampling design.

Sampling Criterion• Degree of Accuracy – Depends on the researcher’s

tolerance for errors in sampling and requirements of the project

• Resources – Depends on the researcher’s financial and human resource constraints

• Time – Depends on the deadline of the project completion

• Advance Knowledge of the Population – Depends on the availability of details of population characteristics

• National vs Local – Depends on the geographic proximity of the population elements

Internet Sampling

Advantages• Allow researchers to reach a large sample rapidly• Sample size requirements can be met quickly• Easier to carry out • Less costlyDisadvantages• Lack of computer ownership and internet access • Unrepresentative of all target populations

• Volunteer respondents• Unrestricted/convenience samples• Arrive haphazardly• Random selection of sample units is a better option• Done through Pop-up ads• Problem of over representing the frequent visitors to the

site• Can be controlled by several techniques like cookies,

prescreening etc• Valuable if the target population is defined as visitors to a

particular Web site

Web Site Visitors

Panel Samples• Drawing a probability sample from an established

consumer panel or other pre-recruited membership panel

• Yields a high response rate• Easier to select the panelists based on the data of their

previously answered questionnaires• Panelists are compensated for their time with a

sweepstakes, a small cash incentive, or redeemable points, etc

• Allows the company to draw simple random samples, stratified samples, and quota samples

Recruited Ad Hoc Samples• A sampling frame of e-mail addresses on an ad hoc basis• Can be done online or offline• Can be compiled from many sources, including

customer/client lists, advertising banners on pop-up windows that recruit survey participants, online sweepstakes, and registration forms

• Respondents maybe contacted by “snail mail” or by telephone to ask for their e-mail addresses and obtain permission for an Internet survey

• Offline techniques used are random-digit dialing and short telephone screening interviews

Opt-in Lists• To give permission to receive selected e-mail, such as

questionnaires, from a company with an internet presence

• E-mail is sent only to authorized recipients• Each individual has to confirm and reconfirm their

consent to participate in the survey• Unsolicited survey request is treated as spam• High response rate cannot be expected from the

individuals who have not agreed to be surveyed• It can lead to complaints to the Internet Service

Providers and the survey site may be shut down