Population
the entire group of individuals or instances about whom we hope to learn
Sample
a (representative) subset of a population, examined in hope of learning about the population
Sample Survey
a study that asks questions of a sample drawn from some population in the hope of learning about the entire population; Polls taken to assess voter preferences are common these
Bias
any systematic failure of a sampling method to represent its population is this; These sampling methods tend to over-or underestimate parameters; It is almost impossible to recover from this, so efforts to avoid it are well spent
Randomization
the best defense against bias is this, in which each individual is given a fair, random chance of selection
Sample size
the number of individuals in a sample; Determines how well the sample represents the population, not the fraction of the population sampled
Census
a sample that consists of the entire population
Population parmeter
a numerically valued attribute of a model for a population.
Statistic
values calculated for sampled data
Representative
if the statistics computed from the sample accurately reflect the corresponding population parameters
Simple Random Sample SRS
this of sample size n is a sample in which each set of n elements in the population has an equal chance of selection
Sampling Frame
a list of individuals from which the sample is drawn
Sampling variability
natural tendency of randomly drawn samples to differ, one from another
Stratified random sampling
population is divided into subpopulations, or strata, and random samples are then drawn from each stratum; Best if strata are homogeneous, but different from each other
Cluster sampling
entire groups, or cluster, are chosen at random. Selected as a matter of convenience, practicality, or cost; Clusters should be representative of the population, and therefore heterogeneous and similar to each other
Multistage sampling
combine several different types of sampling methods
Systematic sample
individuals are selected systematically from a sampling frame. first number must be random; ex: every 10th person
Pilot
a small trial run
Voluntary Response Bias
individuals can choose on their own whether or not to participate in the sample
Convenience Sample
taken from individuals who are conveniently available
Undercoverage
part of population is less represented
Nonresponse bias
large fraction of those sampled fail to respond; those who respond are not likely to represent the whole population; ex: telephone survey
Response bias
the word of questions that influences a responders answer; ex: "How to you feel about the cost cuts to local zoos that are making animals starve to death?