Population
The entire group of individuals about which we want information
Sample
The part of the population from which we actually collect information; information is used to draw conclusions about the entire population
Sample Survey
A study that uses an organized plan to choose a sample that represents some specific population. We base conclusions about the population on data from the sample.
Convenience Sample
Choosing individuals who are easiest to reach; particularly prone to large bias
Bias
Systematically favors certain outcomes; overestimates or underestimates a value you want to know
Voluntary Response Sample
People decide whether to join a sample based on an open invitation; people with strong opinions respond; particularly prone to large bias
Random Sampling
The use of chance to select a sample; is the central principle of statistical sampling.
Simple Random Sample (SRS)
Basic random sampling method; Every possible sample of a given size has the same chance to be chosen.
Label the members of the population and use random digits to select the sample.
Table of Random Digits
A long string of the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 with these properties:
� Each entry in the table is equally likely to be any of the 10 digits 0 through 9.
� The entries are independent of each other. That is, knowledge of one part of the table gi
Stratified Random Sample
Classify the population into groups of similar individuals, called strata. Then choose a separate SRS from each stratum to form the full sample.
Strata
Groups of individuals in a population that are similar in some way that might affect their responses
ex: Freshmen, sophomores, juniors, seniors
Cluster Sample
Divide the population into smaller groups (clusters), ideally, mirroring the characteristics of the population. Then choose an SRS of the clusters. All individuals in the chosen clusters are included in the sample.
Cluster
Smaller groups with similar characteristics of the population; ex: a homeroom classes assigned alphabetically
Inference
Drawing conclusions about the population based on what we know about the sample
Sampling Error
Mistakes made in the process of taking a sample that could lead to inaccurate information about the population. Bad sampling methods and undercoverage are common types of sampling error.
Sampling Frame
The list from which a sample is actually chosen.
Undercoverage
A sampling error that occurs when some members in the population are left out of the process of choosing the sample
Nonsampling Error
Occurs when participants cannot be reached or the sample data are incorrectly collected, recorded, or analyzed (such as using a defective measurement instrument or copying the data incorrectly).
Nonresponse
A nonsampling error that occurs when a selected individual cannot be contacted or refuses to cooperate
Response Bias
A systematic pattern of incorrect responses in a sample survey
Wording of Questions
Most important influence on the answers given to a survey. Confusing or leading questions can introduce strong bias, and changes in wording can greatly change a survey's outcome. Even the order in which questions are asked matters