AP Statistics Chapters 12 and 13

Population

The entire group of individuals or instances about whom we hope to learn

Sample

a subset of the population examined in hope of learning more about the population

Sample Survey

A study that asks questions of a sample drawn from some population in the hope of learning something about the entire population

Bias

A systematic failure of a sampling method to represent the entire population; tends to over or under estimate parameters

Types of bias

voluntary/non- response bias, undercoverage, response bias

Randomization

each individual has an even chance of being selected

census

a sample consisting of the entire population

population parameter

a numerically valued attribute of a model (ex: mean income of all employed people in the country)

Statistic

values calculated for the sample data

Representative

a sample whose statistics accurately reflect the corresponding population parameters

Sampling frame

a list of individuals from whom the sample is drawn

Sampling variability

the natural tendency of randomly drawn samples to differ

stratified random sample

a sampling design in which the population is divided into several subpopulations, or strata, and random samples are then drawn from each stratum.

Strata

A homogenous section of the data (one layer of the cake)

Cluster Sample

A sampling design in which entire groups, or clusters, are chosen at random

Cluster

representative of the entire population; heterogeneous and similar to each other

Multistage Sample

Sampling schemes that combine several sampling methods

Systematic sample

selecting individuals systematically from a sampling frame

Pilot

trial run of a survey to check whether questions are clear; reduces errors due to ambiguous questions

Voluntary response bias

when individuals choose whether to participate or not; always invalid

Convenience sample

consists of individuals who are convenient to sample; not representative

Undercoverage

Biases the sample in a way that gives a part of the population less representation than it has in the overall population

Nonresponse bias

when a large fraction of those sampled fails to respond; those who do respond do not represent the entire population.

Response bias

Anything in a survey design that influences responses; ex, wording of questions

Observational Study

a study of data that has not been manipulated

Retrospective study

an observational study in which subjects are selected and then their previous conditions or behaviors are examined

Prospective study

an observational study in which subjects are followed to observe future outcomes

Experiment

A study that manipulates factor levels to create treatments, randomly assigns subjects to these treatment levels, and then compares the responses of the subject groups across treatment levels

Factor

A variable whose levels are manipulated by the experimenter. Experiments attempt to discover the effects that differences in factor levels may have on the responses of the experimental units

Response variable

A variable whose values are compared across difference treatments

Experimental units

individuals on whom the experiment is performed; called subjects or participants if they are human

Level

The specific values that the experimenter chooses for a factor

Treatment

the controlled circumstance applied to randomly assigned experimental units. the different levels of a single factor

Principals of experimental design

Control, randomization, replication, blocking

Statistically significant

when an observed difference is too large for us to believe it occurred naturally

control group

the experimental units assigned to a baseline treatment level, typically the default treatment or a placebo treatment--basis for comparison

blinding

any individual associated with an experiment who is not aware of how subjects have been allocated to treatment groups

single and double blind

blinding those who influence results (single), or that and those who evaluate the results (double)

placebo

a treatment known to have no effect; administered to maintain the same conditions within groups

placebo effect

the tendency of many human subjects to show a response when administered a placebo

blocking

when groups of experimental units are similar, you gather them into blocks: isolates the variability attributable to the differences between the blocks so we can see the affect of the treatments more clearly

matching

in a retrospective or prospective study, subjects who are similar in ways not under study are matched and compared with each other on the variables of interest

Designs

randomized and blocked

Confounding variable

when the levels of one factor are associated with the levels of another factor in such a way that their effects cannot be separated, the levels are confounded