Population
The entire group of individuals we want information from
Census
Collects data from every individual in the population
Sample
A subset of individuals in the population form which we actually collect data
Choosing a Sample
Step 1: Define the population we want to describe
Step 2: Say exactly what we want to measure
Step 3: Decide how to choose a sample from the population
Convenience Sample
Choosing individuals from the population who are easy to reach
Bias
If it would consistently underestimate or consistently overestimate the value you want to know
Voluntary Response Sample
Consists of people who choose themselves by responding to a general invitation (people with strong opinions)
Random Sampling
Involves using a chance process to determine which members of a population are included in the sample
Simple Random Sample
Is chosen in such a way that every group of n individuals in the population has an equal chance to be selected as the sample
How to Choose an SRS
Step 1: Label. Give each individual in the population a distinct numerical label from 0/1 to N.
Step 2: Randomize. Use a random number generator or data table to obtain n different integers from 0/1 to N.
Stratified Random Sample
Start by classifying the population into groups of similar individuals called strata then choose a separate SRS in each stratum and combine these SRSs to form the sample
Cluster Sample
Start by classifying the population into groups of individuals that are located near each other, called clusters, then choose an SRS of the clusters (all individuals are included in sample)
Undercoverage
Occurs when some members of the population cannot be chosen in a sample (not present)
Nonresponse
Occurs when an individual chosen for the sample can't be contacted or refuses to participate
Response Bias
A systematic pattern of incorrect responses in a sample survey leads to response bias
Wording of Questions
Is the most important influence on the answers given to a sample survey (wording of question is changed to include less or more info)