AP Stats - Unit 1B

Sample Size

The number of individuals in a sample

Census

The sample is the entire population

Simple Random Sampling (SRS)

Sampling method in which all possible samples of a given size (n) have an equally likely chance of being chosen

Stratified

Population divided into homogeneous groups (strata), then SRS takes place within each strata

Cluster

Population divided into heterogeneous groups (based on proximity to one another) are chosen at random, then SRS takes place within each cluster

Systematic

A random starting point is chosen, and from there, individuals are chosen for the sample based on a systematic method of selection (a starting point among the first 'k' individuals is selected, then we keep finding the 'kth' term after this)

(Note: We typically divide the population total by the sample size to determine 'k,' the amount by which you want to keep counting as you sample)

Multistage

Sampling done via a combination of methods (for example, a poll may be taken in stratified regions of the U.S.A., then from these regions, a random sample of cities is chosen, and from there, cluster sampling is done at these cities)

Convenience Sampling

Sample of individuals who are easily available

Voluntary Response Sampling

Survey is open to EVERYONE, and thus, anyone can respond

Bias

Errors in the sampling method or sampling design that consistently lead to overestimates or underestimates of the true value you are trying to measure NOTE: When describing how the design of a sample survey leads to bias, you must include TWO THINGS:

• Describe how members of the sample may differ from those of the population

• Explain how this difference would lead to an overestimate or underestimate

Sampling Error

The natural variation that occurs among stats from taking different samples of a population

Nonresponse

Subjects within a sample are invited to respond but either can't be contacted or refuse to participate

Nonresponse Bias

Can occur when nonresponse is present because the portion that is not responding may have differing opinions/viewpoints than those who are responding, making it difficult to understand which opinions/viewpoints are representative of the population.

Voluntary Response Bias

Can occur when those who are willing to respond may have differing opinions/viewpoints than those who are not responding, making it difficult to understand which opinions/viewpoints are representative of the population.

Undercoverage

Part of population is consistently less sampled than others due to improper sampling frame (group(s) are systematically left out of the sample)

Response Bias

Occurs when responses to a survey question consistently differ from the truth in some way (Note: This occurs when the phrasing of a question is poor or possibly leading, or if the surveyor is asking in

Observational Study

Observes items or individuals and measures variables of interest, but does NOT impose treatments

Retrospective Study

Observational units are selected at a point in time and data about them are gathered from the past

Prospective Study

Observational units are selected at a point in time and data about them are gathered both at that time and into the future

Survey

An observational study in which data are collected from people by asking them a common set of questions

Control Group

Used to provide a baseline for comparing the effects of other treatments. Depending on the purpose of the experiment, a control group may be given an inactive treatment, an active treatment, or not treatment at all.

Placebo

A treatment that has no active ingredient, but is otherwise like the other treatments

Placebo Effect

Describes the fact that some subjects in an experiment will respond favorably to any treatment, even an inactive treatment. In order for subjects to not know which treatment they are receiving, blinding can be used.

Single blind Experiment

Either the subjects or the people who interact with them and measure the response variable don't know which treatment a subject is receiving (typically the subjects are the group to which blinding applies)

Double blind Experiment

Neither the subjects nor those who interact with them and measure the response variable know which treatment a subject is receiving.

Random Assignment of Treatments

Treatments are assigned to experimental units (or vice versa) using a chance process

Completely Randomized Design

Experiment in which experimental units are assigned to treatments completely at random

Replication

Refers to having multiple experimental units assigned to each treatment. This is not the same as repeatability: to repeat our experiment and compare results over and over.

Direct Control

Keeping the values of some extraneous variables the same for all experimental units

Extraneous Variables

Variables other than the explanatory variable that may have an effect on the response variable in a statistical study/experiment.

Confounding Variables

Extraneous variables that are associated with both the explanatory variable AND the response variable. The presence of a confounding variable makes it difficult to determine whether changes in the explanatory variable cause changes in the response variable.