1. when i give you the signal, you will have 10 seconds to look at a slide and make a guess as to...
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Introduction to Sampling
1. When I give you the signal, you will have 10 seconds to look at a slide and make a guess as to the average number of m&m’s per pile. Do not use pencil or paper… just guess.
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Class Survey “Draft”On Word Document
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Chapter 5.1 Designing Samples
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Population and Sample
Population-entire group of individuals we want information about.
Sample-part of the population to represent the population
Population Sample
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Study Methods
Observational To gain information w/out influence
Experiment Imposes some treatment on individuals
to observe response. Use for cause and effect
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Observational Methods
Sampling Used to study part of a population to
gain information of the whole population
Census Attempts to contact every individual in
the population.
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Types of Sampling Designs
Voluntary response People choose to participate Extremely Biased “bad design”
Convenience Sample Based on easy access with out looking at
representation of the whole population▪ Example: You want to know if Kimball students like
Math so you ask your math class. “bad design”
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Practice
Exercise 5.1-5.8
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Types of Sampling Designs (cont.)
Simple Random Sample (SRS) Random, everyone has a chance of
being picked This is a good Sampling method
Stratified Random Sampling Used when you want groups to be
equally represented.
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How to create an Simple Random Sample (SRS)
Random, everyone has a chance of being picked Step 1: Label: Assign an numerical value to
all the individuals Step 2: Random Assignment ▪ Use the random number table (Table B) ▪ Random number generator (using the calculator)▪ MathPRB#5:RandIntenter (1,33)▪ Seed
5rand Seed #, store, math, PRB, option 1:rand
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Stratified Random Sample
Stratified Random Sampling is NOT SRS Step 1: Separate the population into similar
groups called strata▪ For equal representation
Step 2: SRS each strata, these SRSs form your sample
Choose strata based on facts known before the sample is taken
Ex: A population of election districts might be divided into urban, suburban, and rural strata.
Every group is represented.
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Multistage Sampling Design
Randomly choose stage 1 strata Random States in the country
Randomly choose stage 2 strata Random cities in the states
And so on until you get your sample size.
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Systematic Sample Every 5th person to walk by is interviewed
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“Random” is the key to a good sample
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Practice
Exercise 5.9-5.11
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Cautions about sample surveys
Undercoverage A group of the population is left out If you use a phone directory to get a sample.
You miss out on… Nonresponse
No answer, do not mail back etc. Left out of the representation
Response Bias Participants are not telling the truth
Wording of questions
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Why Sample?
We want to make inferences about the population as a whole population.
We cant afford to talk to everyone.
Note: Two samples following the same design will give you different results. Each one is an estimate of the population.
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How to get the best Estimate
Large random samples give more accurate results than smaller samples.
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Assignment
Read the summary on Pg.285 Exercise 5.19-5.29odd, 26 Plan your Data Collection Sample
You need to write your plan on a binder paper▪ Goal: Describe Population and purpose of survey.▪ Sample: State Sampling Design, size and plans for
implementation.▪ Foreseen Bias: What are they, Give details on how they apply (ex:
undercoverage-who was left out), how do they impact your results.
Turn in raw data Display data visually and Describe Summarize your experience collecting the data▪ Cautions experienced- restate the predicted and additional ones. ▪ Any surprises
Due Thursday
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