L7a Sampling
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Transcript of L7a Sampling
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Chapter 7
Sampling and
Sampling Distributions
David Chow
Oct 2014
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Learning Objectives
In the first part of this chapter, you will learn:
Different sampling methods
How to evaluate survey worthiness
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An Example of Sampling
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Why Sample?
Instead of selecting every item inthe population (ie, doing a census),
A sample is
less costly to select, and
more practical to analyze
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Types of Samples
In a nonprobability sample, items included are chosen
without regard to their probability of occurrence Eg: conveniencesampling
Eg:judgmentsample, you get the opinions of pre-selected
experts in the subject matter
Types of
Samples
Nonprobability
Samples
Probability
Samples
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Types of Samples
In aprobability sample, items in the sample are chosen
on the basis of known probabilities.
Probability Samples
Simple Random SystematicStratified
Cluster
each possible sample of size n
has the same probability of
being selected
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Simple Random Sampling
Every individual or item from the frame has an equalchance of being selected
Selection may be with or without replacement Samples obtained from table of random numbers or
computer random number generators.
Excel command:
=RAND(), OR=RANDBETWEEN(lower l imi t, upper limit)
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Systematic Sampling
Decide on sample size: n
Divide all N individuals into groups of k
individuals: k = N/n
Randomly select one individual from the 1stgroup
Select every kthindividual thereafter
Eg: Suppose you were sampling n = 5 from a population of N = 50
Step 1: Divide the population into n groups, each with k = 50/5 = 10 individuals
Step 2: Randomly select a member from group 1, say individual 3
Step 3: Select every kthindividual thereafter. The sample is {3, 13, 23, 33, 43}
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Stratified Sampling
Divide population into two or more subgroups (calledstrata)
By some common characteristic like age, gender, industry, etc
A simple random sample isselected from each stratum
Samples from subgroups arecombined into one
Eg: Sampling CU students bystratifying across ____
Arome Bakerys mango mousse cakeFigure
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Cluster Sampling
Population is divided into several clusters, each
representative of the population
A cluster is a small-scale version of the population
A simple random sample of clusters is selected
Eg: Election exit polls, where certain election districts are
selected and sampled
Eg: How to divide CU students into cluster?
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Comparing Sampling Methods
1. Simple random sample and systematic sample
Simple to use
May not be a good representation of the population
2. Stratified sample
Ensures representation of individuals across the entire
population
3. Cluster sample
More cost effective, but less efficient (i.e., a ____ sample
needed for the same level of precision).
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Evaluating Survey Worthiness
Any bias
Sometimes, we want to know who conducts the survey
Sample size? Four common errors
1. Coverage error, or selection bias
2. Non-response error
3. Measurement error
4. Sampling error
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Types of Survey Errors1. Coverage error or selection bias
Exists if some groups are excluded from the frame and have no chanceof being selected.
2. Non-response error
People who do not respond may be different from those who do respond.
3. Measurement error
Due to weaknesses in question design, respondent error, andinterviewers impact on the respondent.
4.Sampling error Variation from sample to sample, cannot be fully eliminated.
Which of these errors are (a) inevitable, (b) fatal?
Remember the saying:Garbage in, ____ !
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Eg: A Poll on Occupy CentralBy the largest political party, the pro-Beijing Democratic
Alliance for the Betterment and Progress of Hong Kong
Potential Problems
Sampling: 70% of the 1,097
respondents are aged 50+
Loaded Question: Are you worried
that Central will be paralyzed & HK
will suffer severe economic loss?
The poll did catch considerable attention from the media, though:
70pc frown on Occupy Central: DAB poll (SCMP, 2013-04-10)
Most HK residents oppose 'Occupy Central (China Daily Asia, 2013-04-08)
DAB says majority against Occupy Central plan (Standard, 2013-04-09)