AP STATISTICS

Advantage of using a stratified random sample over a SRS

Stratified random sampling guarantees that each of the strata will be represented. When strata are chosen properly, a stratified random sample will produce better (less variable/more precise) information than an SRS of the same size. Done for comparison purposes.

Bias

The systematic favoring of certain outcomes due to flawed sample selection, poor question wording, under coverage, nonresponse, etc.
Bias deals with the center of a sampling distribution being "off"!

Single Blindness

when the subject OR the researcher does not know which treatment is being administered

Double Blindness

both subject and researcher do not know

Census

population count+An attempt to reach the entire population

Cluster Sample

a sampling design in which entire groups are chosen at random. Usually selected as a matter of convenience, practicality or cost. each cluster should be diverse in character and representative of the population, so all the groups should be similar to each other

Convenience sample

sample consisting of the individuals who are conveniently available

Does ___ CAUSE ____?

Association is NOT Causation!
An observed association, no matter how strong, is not evidence of causation. Only a well-designed, controlled experiment can lead to conclusions of cause and effect.

(Completely Randomized Design)

All experimental units are allocated at random among all treatments

Randomized Block Design

Experimental units are put into homogeneous blocks. The random assignment of the units to the treatments is carried out separately within each block.

Matched Pairs

A form of blocking in which each subject receives both treatments in a random order or the subjects are matched in pairs as closely as possible and one subject in each pair receives each treatment, determined at random.

Experiment

remember to identify the treatment.
In an observational study researchers make no attempt to influence the results.

Observation Study

researched IMPOSE a treatment upon the experimental units.

Goal of Blocking

The goal of blocking is to create groups of homogeneous experimental units.

Benefit of Blocking

The benefit of blocking is the reduction of the effect of variation within the experimental units. Helps control lurking or confounding variables. (context)

Matching

Any attempt to force a sample to resemble specified attributes of the population

Multistage sample

sampling schemes that combine several sampling methods

Nonresponse bias

Bias introduced to a sample when a large fraction of those sampled fails to respond. Those who do respond are likely to not represent the entire sample. Voluntary response bias is a one form. For example, those who are at work during the day won't respond to a telephone survey conducted only during working hours.

Population

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

Population parameter

a numerically valued attribute of a model for a population.

Randomization

the best defense against bias, in which each individual is given a fair, random chance of selection

representative

this kind of sample accurately reproduces the characteristics of the population a researcher is studying

Response bias

Anything in a survey design that influence response. Typically arises from the wording of questions, which may suggest a favored response. Voters, for example, are more likely to express support of "the president" than support of the particular person holding that office at the moment.

Sample

A (representative) subset of a population, examined in hope of learning about the population

Sample Size

the number of individuals in a sample; determines how well the sample represents the population, not the fraction of the population sampled.

Sample Survey

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

Sampling frame

the individuals or clusters of individuals who might actually be selected for inclusion in the sample

Simple Random Sample

Number the entire population, draw number from a hat (every set of n individuals has equal chance of selection)

Stratified

split the population in homogeneous groups, select an SRS from each group

Cluster

Split the population into heterogeneous groups called clusters, and randomly select whole clusters for the sample. Ex. Choosing a carton of eggs actually chooses a cluster (group) of 12 eggs.

Convenience

Selects individual easiest to reach

Voluntary Response

People choose themselves by responding to a general appeal. Those that choose to participate usually feel very strongly one way or the other.

sampling variability

the natural tendency of randomly drawn samples to differ, one from another; the natural result of random sampling.

Statistic, sample statistic

values calculated for sampled data. Those that correspond to, and thus estimate, a population parameter, are of particular interest. For example, the mean income of all employed people in a representative sample can provide a good estimate of the corresponding population parameter. The term "sample statistic" is sometimes used, usually to parallel the corresponding term, "population parameter."

Strata is to __________ as Blocking is to ___________

Surveys, Experiments

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.

Systematic sample

a sample drawn by selecting individuals systematically from a sampling frame

There is no recovery from ....

poorly collected data

Voluntary response bias

when only those that choose to participate, do. Those that participate usually have a strong feeling one way or the other.

Undercoverage bias

when certain groups are left out of the survey (not even in the sampling frame)-usually left out because they may be difficult to reach (ie homeless people may be hard to reach)

selection bias

when one group is selected more heavily than another group.

Unbiased Estimator

The data is collected in such a way that there is no systematic tendency to overestimate or underestimate the true value of the population parameter. (the mean of the sampling distribution equals the true value of the parameter being estimated)

Undercoverage

A sampling scheme that biases the sample in a way that gives a part of the population less representation than it has in the population.

Why use a control group?

A control group gives the researchers a comparison group to be used to evaluate the effectiveness of the treatment(s). Reduces the effect of confounding variables. (gauge the effect of the treatment compared to no treatment at all)