AP Statistics Unit 3: Collecting Data

Observational Study

A study where the researcher observes and measures characteristics without imposing treatments

Retrospective

An observational study that looks back at data that has already occurred

Prospective

An observational study that follows participants forward in time

Experiment

A study in which the researcher imposes different treatments to observe the effect on another variable

Experimental Units

The individuals or objects to which treatments are applied in an experiment

Explanatory Variable

A variable whose levels are manipulated intentionally (old term)

Factor

A variable whose levels are manipulated intentionally (new term)

Response Variable

Outcome from the experimental units that is measured after the treatments have been administered

Levels

The different values or categories of a factor in an experiment

Treatments

Levels or combination of levels of the explanatory variable(s)

Confounding Variable

A variable that is related to the explanatory variable and influences the response variable and may create a false perception of association between the two

Control Group

Collection of experimental units either not given a treatment of interest or given a treatment with an inactive substance (placebo) in order to determine if the treatment of interest has an effect

Bias

Certain responses are systematically favored over others

Blocks

Groups of experimental units that are similar in some way that is expected to affect the response to the treatments

Strata

Subgroups within a population that share similar characteristics / Homogeneous grouping

Placebo

A treatment that has no therapeutic effect, used as a control in experiments

Placebo Effect

Experimental units have a response to a placebo

Single-Blind Experiment

Subjects do not know which treatment they are receiving OR researchers don't know which treatment subjects are receiving

Double-Blind Experiment

Neither the subjects nor the members of the research team who interact with them know which treatment a subject is receiving

Random Assignment

Balances the effects of confounding variables which allows for a cause and effect conclusion

Completely Randomized Design

Treatments are assigned to experimental units completely at random

Randomized Block Design

Treatments are assigned completely at random within each block

Matched Pairs

An experimental design where participants are paired based on similar characteristics and each pair is randomly assigned a different treatment (Can also be done by assigning both treatments to one subject in a randomly assigned order)

Census

Selects all items or subjects in a population

Simple Random Sample

A sample where every individual has an equal chance of being selected

Stratified Random Sample

A sample that involves dividing the population into groups and then taking a simple random sample from each group

Cluster Random Sample

A sampling method where the population is divided into groups and a simple random sample is used to select entire groups

Systematic Random Sample

A sample obtained by selecting every k-th individual from a list of the population after randomly selecting the first individual

Convenience Sample

A sample that is taken from a group that is easy to reach

Convenience Bias

When a sample is comprised entirely of easily obtainable subjects, the sample will typically not be representative of the population

Volunteer Sample

A sample consisting of individuals who volunteer to participate

Voluntary Response Bias

When a sample is comprised entirely of volunteers or people who choose to participate, the sample will typically not be representative of the population

Undercoverage Bias

When part of the population has a reduced chance of being included in the sample, the sample will typically not be representative of the population

Nonresponse Bias

Individuals chosen for the sample for whom data cannot be obtained (or who refuse to respond) may differ from those for whom data can be obtained

Response Bias

Problems in the data gathering instrument or process

Statistic

A numerical measurement describing some characteristic of a sample

Parameter

A numerical measurement describing some characteristic of a population

Population

The entire group of individuals or subjects of interest

Sample

A subset of the population

Random Sample

A sample that is selected randomly from the population which allows for a generalization to be made to the population

Statistically Significant

A result that is unlikely to have occurred by chance, indicating a real effect

Sampling Error

The difference between the sample statistic and the population parameter due to random chance which can be decreased by increasing sample size

Cluster

Subgroups within a population that share different characteristics / Heterogeneous grouping

Sample Survey

Type of observational study that collects data from a sample in an attempt to learn about the population from which the sample was taken