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.

Class Survey “Draft”On Word Document

Chapter 5.1 Designing Samples

Population and Sample

Population-entire group of individuals we want information about.

Sample-part of the population to represent the population

Population Sample

Study Methods

Observational To gain information w/out influence

Experiment Imposes some treatment on individuals

to observe response. Use for cause and effect

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.

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”

Practice

Exercise 5.1-5.8

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.

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

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.

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.

Systematic Sample Every 5th person to walk by is interviewed

“Random” is the key to a good sample

Practice

Exercise 5.9-5.11

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

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.

How to get the best Estimate

Large random samples give more accurate results than smaller samples.

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