AP Statistics: Chapter 4 - Collecting Data

Census

a study that obtains data from every member of a population

Population

the entire group of individuals that you want information about

Sample

a subset of the population that you collect data from and is meant to represent the population

Simple Random Sample

every individual has an equal chance of being selected for the sample, and every possible sample combination has an equal chance of being chosen

Stratified Random Sample

a strata is a group of individuals that share certain characteristics thought to be associated with the variables being measured in the study. This sampling method randomly selects individuals from each strata

Cluster Sample

a cluster is a group of individuals in the population that are located near each other. This sampling method randomly chooses clusters of individuals to be included in the sample

Convenience Sample

a sample that selects individuals from the population who are easy to reach

Bias

the tendency of a sample to systematically overestimate or underestimate the value you want to know

Voluntary Response Bias

the individuals who respond are likely to have the same opinion or feel strongly on a certain topic, leading to bias

Nonresponse Bias

bias introduced to a sample when a large fraction of those sampled fails to respond. Can lead to bias because usually the results obtained will be skewed due to missing data

Response Bias

participants answer questions falsely. Several factors can lead to this bias, such as the wording of questions, gender, and age. Individuals may lie in their response or provide an inaccurate response

Undercoverage

occurs when some members of the population are less likely to be chosen or cannot be chosen in a sample

Inference

the process of drawing conclusions about an underlying population based on a sample or subset of the data

Wording of Questions

the most important influence on the answers given to a survey. Confusing or leading questions can introduce strong bias, and changes in wording can greatly change a survey's outcome. Even the order in which questions are asked matters

Observational Study

observes individuals and measures variables of interest but does not attempt to influence the responses

Experiment

a researcher deliberately imposes some treatment on individuals to measure their responses

Confounded

occurs when two variables are associated in such a way that their effects on a response variable cannot be distinguished from each other. It becomes difficult to tell which variable is causing the change in the response variable

Treatments

the experimental conditions imposed on the experimental units by the experimenter. If an experiment has several explanatory variables, a treatment is a combination of specific values of these variables

Factors

a variable that is manipulated and may cause a change in the response variable. This often occurs when there are multiple explanatory variables

Experimental Units

the object to which a treatment is randomly assigned

Subjects

human experimental units

Comparison

an experimental design that compares two or more treatments. Can use a control group to compare with a group that receives the treatment

Random Assignment

assigning experimental units to treatments randomly helps to create roughly equivalent groups of experimental units by balancing the effects of other variables among the treatment groups

Control

keep other variables that might affect the response the same for all groups. Helps to avoid confounding and reduces variability in the response variable

Replication

use enough experimental units in each group so that any differences in the effects of the treatments can be distinguished from chance differences between groups

Blocking

a group of experimental units that are known before the experiment to be similar in some way that is expected to affect the response to the treatments

Placebo

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

Placebo Effect

some subjects in an experiment will respond favorably to any treatment, even an inactive treatment

Double-blind

neither the subjects nor those who interact with them and measure the response variable know which treatment a subject received

Single-blind

either the subjects don't know which treatment they are receiving or the people who interact with them and measure the response variable don't know which subjects are receiving which treatment

Statistically Significant

when the observed results of a study are too unusual to be explained by chance alone

Completely Randomized Design

the experimental units are assigned to the treatments completely by chance

Randomized Block Design

the random assignment of experimental units to treatments is carried out separately within each block

Matched Pairs Design

a type of randomized block design used for comparing two treatments that uses blocks of size 2, pairs. In some cases, two very similar experimental units are paired and the two treatments are randomly assigned within each pair. In others, each experimental unit receives both treatments in a random order