sampling errors

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Sampling Designs and Sampling Procedures Chapter 16 Presented by: Qadiruddin Sufi Sameen Ejaz Tooba Farrukh

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Page 1: Sampling Errors

Sampling Designs and

Sampling Procedures

Chapter 16

Presented by:

Qadiruddin SufiSameen EjazTooba Farrukh

Page 2: Sampling Errors

Under discussion

Sampling terminology Sampling concepts Random vs. non random sampling Types of systematic errors Types of probability & non probability

samples Appropriate sample design

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SAMPLING DESIGNS

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SAMPLING TERMINOLOGY

SAMPLINGThe process of using a small number of items or

parts of a larger population to make conclusion about the whole population.

SAMPLEA subset or some part of a larger population.

POPULATION(UNIVERSE)A complete group of entities sharing some common

set of characteristics.

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POPULATION ELEMENTAn individual member of a specific population.

CENSUSAn investigation of all the individual elements

making up a population.

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WHY SAMPLE?

PRAGMATIC REASON

Sampling cuts costs, reduces labor requirements and gather vital information quickly.

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ACCURATE AND RELIABLE RESULTS

If properly selected are sufficiently and accurate in most cases. If the elements of a population are quite similar only a small sample is necessary to accurately portray the characteristics of interest.

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DESTRUCTION OF TEST UNITS

If there is a finite population and everyone in the population participates in the research and cannot be replaced, no population element remain to be selected as sampling units. The test units have been destroyed.

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PRACTICAL SAMPLING CONCEPTS

Researchers must make several decisions before a sample is taken.

Define the target population

Select a sample frame

Determine if a probability or non probability sampling method will be

chosen

Plan procedure for selecting sample units

Determine sample size

Select actual sampling units

Conduct fieldwork

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STEP 1 DEFINING THE TARGET POPULATION

TARGET POPULATIONspecific, complete group relevant to the research project.

The question “To whom do we want to talk?”must be answered .It may be users, on users ,recently hired employees ,doctors. To implement the sample in the field, tangible characteristics should be used to define the population.

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STEP 2 SELECT A SAMPLE FRAME

SAMPLING FRAME The list of elements from which a sample

may be drawn, also called working population.The sampling frame is also called working

population because it provides the list that can be worked with operationally.

MAILING LISTS A list of the names, addresses and phone numbers

of specific populations.

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REVERSE DIRECTORY A directory similar to a telephone directory in

which listings are by city and street address or by telephone numbers rather than alphabetical by last name.

SAMPLING FRAME ERROR Error that occurs when certain sample elements

are not listed or available and are not represent in the sampling frame.

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SMAPLING UNITS A single element or group of elements subject to

selection in the sample

PRIMARY SAMPLING UNIT(PSU) A unit selected in the first stage of sampling.

SECONDARY SAMPLING UNIT A unit selected in the second stage of sampling

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Sampling Errors

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Random Sampling Error

Statistical fluctuation that occurs because of chance variation in the elements selected for a sample is called random sampling error

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Sampling error- function of sample size

Sampling error

Population Population

Sampling error

Sample Sample

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Systematic / Non sampling Error

Errors resulting primarily from some imperfect aspect of research design or from a mistake in the execution of research

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Less than perfectly representative sample

Random sampling error, systematic error combine to give less than perfectly representative sample.

Systematic errori. Sampling frame errorii. Non response error

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Non response error Statistical difference between a survey

that included only those who responded and a survey that includes also those who failed to respond

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Actual sample is drawn from a population different from the ideal.

Total Population

Planned Sample

Actual Sampl

e

Sampling frame

Sampling frame error

Random sampling error

Non response error

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Probability vs. Non Probability Sampling

Probability is not known

Each element chosen on basis of convenience or judgment

Best for research purposes

Every element in population has known probability

Each has non zero probability

Mostly preferred over non probability sampling

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Non probability sampling

Convenience sampling

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Judgment sampling Experienced individual selects the

sample based upon some appropriate characteristic of the sample members

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Quota sampling Ensures that certain characteristics of

a population sample will be represented to the exact extent that the investigator desires.

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Sample showing different student categories

punjabsindhbalochistanpeshawar

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Snowball sampling Technique used to locate members of

rare population by referrals

Reduced sample sizeReduced cost

Bias may enter

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Probability sampling

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Probability Sampling

Chance based selection procedure

Probability Sampling techniques:Simple random sampling

Systematic sampling

Stratified samplingProportional versus disproportional sampling

Cluster sampling

Multistage area sampling

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Simple Random Sampling

This assures that each element in the population has an equal chance of being included in the sample

Systematic Sampling

A procedure in which an initial starting point is selected by random process and then selects item at a pre-selected interval

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Stratified sampling

A sub-sample is drawn utilizing simple random sampling within each stratum; more or less equal on some characteristics

Proportional stratified sampleNumber of sample in each stratum is in proportion to the population size at that stratum

Disproportional stratified sampleSample size for each stratum is allocated according to the analytical considerations

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Cluster sampling

The primary sampling unit is not the individual element in the population but a large cluster of elements

Area Sampling where the primary sampling unit is a geographic area

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Multistage Sampling

Sampling that involves using a combination of other probability sampling techniques

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Sample Design Selection

Degree of Accuracy Resources Time Advanced knowledge National versus Local Project Need for Statistical Analysis

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Internet Sampling

Web Site Visitors Panel Sampling Recruited Ad Hoc samples Opt-In Lists

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YOUR TURN NOW

AN investigation of all the individual elements making up a population is called

1. Sample2. Census3. Population element4. Sampling unit

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The list of elements from which a sample may be drawn called

1. Target population2. Sample size3. Sample frame4. Random Sampling

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Which type of non probability sampling technique has more likelihood of having a self selection bias

1. Convenience 2. Judgment3. Quota4. snowball

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What are the two types of systematic errors we have covered

1. Sampling frame and sample bias2. Sampling frame and non response

error3. Non response and non sampling error4. Sampling frame and response bias