Lecture 30sampling and field work

ByAziza Munir

Learning Objectives

• Sampling design and sampling procedures• Sampling terminology• What is sample?• Practical sampling concepts• Random sampling and non sampling error• What should be appropriate sampling design?

Sampling

The process of using a small number of items or parts of large population to make conclusions about the whole populationAlthough sampling has a common place in daily activities, but most of these familiar concepts are not scientific in nature. The understanding of the concept might be intuitive, but actually it has a complex procedure and central importance in business research and data collection, which requires in depth examination

Sampling Terminology

• Sample:The process of sampling involves any procedure using a small number of items or parts of the whole population to make conclusions regarding the whole population. A sample is a subset or some part of a larger population.• Population:Population or universe is a complete group of entities, sharing some common set of characteristics

• Population Element:An individual member of a specific population• Census:An investigation of all the individual elements making up a population. A total enumeration rather than a sample

Why Sample?

• Pragmatic Reason:Cost constraintsTime restrictionLabor limitations• Accurate and reliable results• Destruction of test resultsTo reduce the number of destructions if conducted on perishables or destroyable items

Practical Sampling ConceptsDefine the target population

Select Sampling frame

Select amongst probability and non probability sample

Select sampling units

Determine sample size

Select Actual Sampling Units

Conduct Field Work

Define Target Population

• The specific complete group relevant to the research project

• Comic book example

Sampling Frame

• A list of elements from which a sample may be drawn, also called working population

• A list which can be worked operationally• Maps and Ariel photographs maybe taken as

sample frame if actual access is not possible

Mailing List

• A list providing all names, addresses, and contact numbers of specific population

• Catalog• Reverse Directory:A directory similar to a telephone directory in which listings may be found by city and street addresses and contact numbers rather than in alphabetical order• Exists in multiple formats• Provide demographic data• Compiled by a researcher

Sampling Frame Error

• Error that occurs when certain sample elements are not listed or available and are not represented in the sampling frame

Sampling Units

• A single element or group of elements subject to selection in the sample

Primary sampling Unit• A unit selected in the first stage of samplingSecondary Sampling Unit• A unit selected in the second stage of

sampling

Random Sampling Error and Non Sampling error

• Random sampling error:The difference between the result of sample and the result of census, conducted using identical procedures; a statistical fluctuation that occurs because of chance variation in the elements selected for a sample• Non sampling errorError resulting from some imperfect aspect of the research design that causes response error or from mistake in execution of the research, error that come from such sources as sample bias, mistakes in recording responses, and non responses from persons not contacted and refusing to participate

Non response error

• The statistical difference between a survey, that includes only those who responded and a survey that include those who had not responded

Probability VS Non Probability Sampling

• Non Probability SamplingJudgment/purposive SamplingConvenience SamplingQuota SamplingSnowball sampling

Probability Sampling

• Simple random sampling• Stratified sampling• Systematic samplingProportional/dis proportional sampling• Cluster sampling

Simple Random Sampling

• A sampling procedure that assures each element in the population, an equal chance of being included in the sample

Systematic Sampling

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

• Sampling intervalThe number of population elements between the units selected for sample• PeriodicityA problem that occurs in systematic sampling when original list has a systematic pattern

Stratified Sampling

• A probability sampling procedure in which subsamples are drawn from samples within different strata that are more or less equal on some characteristics

• Proportional stratified sample:A stratified sample in which the number of sampling units drawn from each stratum is in proportion to the relative population size of that stratum

• Non proportional stratified sample:The sample size for each stratum is allocated on analytical consideration• Optimal allocation:A sampling procedure in which both the size and variation of each stratum are considered when determining sample size for stratum

Cluster Sampling

• An economically efficient sampling technique in which the primary sampling unit is not the individual element in population, but a large cluster of elements

• Area sample• Multistage area sample

Appropriate sample design

• Degree of accuracy• Resources• Time• Advance knowledge of population• National and local projects• Need for statistical analysis

Concluding Remarks

• Sample, population and census• Issues regarding identification of target

population and selection of sampling frame• Probability and non probability sampling• How to analyze appropriate sample design