chapter 10 sampling: theories, designs and plans

Chapter 10

Sampling: Theories, Designs

and Plans

Chapter 10

Sampling: Theories, Designs

and Plans

Sampling Terms

• Sampling - selecting a small number of elements (sample) from a larger defined group (population).

• Census - data is collected from every member of the target population.

• Sampling - selecting a small number of elements (sample) from a larger defined group (population).

• Census - data is collected from every member of the target population.

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Population – everyone in the universe

Elements – specific units of interest in the population.(They can be people, products, stores, etc.)

Defined target population – population of interest in the study

Sampling units – individual elements chosen in the sample

Sampling frame – list of potential population elements

Other Sampling Terms

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Sampling Error – any type of bias that results from

mistakes in either the selection process of sampling units or in determining the sample size.http://www.statcan.ca/english/edu/power/ch6/sampling/sampling.htm#samplesize

Sampling Error – any type of bias that results from mistakes in either the selection process of

sampling units or in determining the sample size.http://www.statcan.ca/english/edu/power/ch6/sampling/sampling.htm#samplesize

Types of Errors

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Nonsampling Error – bias that occurs in a research study regardless of whether a sample or census is used; e.g., bias caused by measurement errors,

response errors, coding errors, etc.

Nonsampling Error – bias that occurs in a research study regardless of whether a sample or census is used; e.g., bias caused by measurement errors,

response errors, coding errors, etc.

Probability Simple random Systematic random Stratified random Cluster sampling

Nonprobability Convenience

sampling Judgment sampling Quota sampling Snowball sampling

Types of Sampling Methods

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1. Obtain a list of units that contains an acceptable frame of the target population.

2. Determine the number of units in the list and the desired sample size.

3. Compute the skip interval.4. Determine a random start point.5. Beginning at the start point,

select the units by choosing each unit that corresponds to the skip interval.

Steps in Drawing a Systematic Random Sample

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Selecting a Stratified Random Sample – Three Basic Steps –

(3) Combine samples from each stratum into a

single sample.

(3) Combine samples from each stratum into a

single sample.

(2) Select random samples from each

stratum.

(2) Select random samples from each

stratum.

(1) Divide population into homogeneous

subgroups.

(1) Divide population into homogeneous

subgroups.

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1. Identify target population and determine

clustering factors. (people type, store type, etc).

2. Determine the number of units in each cluster.

3. Determine the number of units in each cluster are needed to represent the cluster.

4. Randomly select units within each cluster.

5. Combine cluster samples into one study sample.

Different than stratified because of the increased need for knowledge of the population and cluster groups to be able to accurately complete it.

Steps in Drawing a Cluster Sample

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Factors to Consider in Sample

Design

Research objectivesResearch objectives Degree of accuracyDegree of accuracy

ResourcesResources Time frameTime frame

Knowledge abouttarget population

Knowledge abouttarget population Research scopeResearch scope

Statistical analysis requirementsStatistical analysis requirements

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Variability of the population characteristic under investigation.

Level of confidence desired in the estimate, most often 95% (within 2 SD).

Degree of precision desired in estimating the population characteristic, e.g. sampling error = +/- 2.

Other . . . o Formulaso Rules of Thumb

Factors Affecting Sample Size

http://www.surveysystem.com/sscalc.htm Sample Size Calculator

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1. Define the Target Population

2. Select the Data Collection Method

3. Identify the Sampling Frame(s) Needed

4. Identify the Appropriate Sampling Method

5. Determine Sample Sizes and Contact Rates

6. Create Plan for Selecting Sampling Units

7. Execute the Plan

Steps in Developing a Sampling Plan

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