Ap stats Chapter 4 - designing studies

Population

The entire group of individuals about which we want information

Sample

The part of the population from which we actually collect information.

Convenience Sample

Choosing individuals who are easiest to reach... almost guaranteed to show bias.

Bias

Consistently overestimating or underestimating the value you want to know.

Voluntary Response Sample

Consists of people who choose themselves by responding to a general appeal. Show bias because people with strong opinions are the most likely to respond.

Random Sampling

Use of chance to select a sample. This is the central principle of statistical sampling.

Simple Random Sample (SRS)

The simplest way to use chance to select a sample e.g. place names in a hat and draw names. (n) individuals from the population chosen in such a way that every set of (n) individuals has an equal chance to be the sample actually selected.

Table of Random Digits

A long string of the digits 0,1,2,3,4,5,6,7,8,9 used to select random samples or perform chance simulations

Strata

Groups of similar individuals

Stratified Random Sample

Classify the population into strata and then choose a separate SRS in each stratum and combine to form a full sample.

Cluster Sample

Selection of whole, pre-existing groups of individuals that mirror the characteristics of the population.

Inference

The process of drawing conclusions about a population on the basis of sample data

Sampling Frame

List of individuals from which a sample is drawn.

Undercoverage

Occurs when some groups in the population are left out of the process of choosing the sample.

Nonresponse

Occurs when an idividual chosen for the sample can't be contacted or refuses to participate. It is the single biggest problem for sample surveys.

Response Bias

Caused by a systematic pattern of incorrect responses in a sample survey

Observational Study

A sample survey where individuals are observed and variables of interest are measured w/o an attempt to influence the response.

Experiment

Deliberately imposes some treatment on individuals to measure their responses. They are the only source of fully convincing data when your goal is to understand cause and effect.

Response Variable

Measures the outcome of a study

Explanatory Variable

Helps to explain or influence changes in a response variable.

Confounding

Occurs when 2 variables are associated in such a way that their effects on a response variable cannot be distinguished from each other.

Treatment

A specific condition applied to the individuals in an experiment.

Experimental Units

Collection of individuals to which treatments are applied. If human, they are called subjects.

Factors

The explanatory variables in an experiment

Random Assignment

Experimental units are assigned to treatments at random, using some sort of chance process

Completely Randomized Design

Treatments are assigned to all experimental units completely by chance

Control Group

Provides a baseline for comparing the effects of the other treatments.

The 4 principles of Experimental Design

1) Comparison 2) Random Assignment 3) Replication 4) Controls

Replication

Using enough experimental units to distinguish a difference in the effects of the treatments from chance variation.

Placebo

Does not have an active ingredient... e.g. sugar pill

Placebo Effect

When a fake treatment results in subjects reporting a response expected from the real treatment

Double-Blind

Neither the subjects nor the researchers know which treatment the subjects received

Single Blind

Either the subjects or the researchers do not know which treatment the subjects received

Statistically Significant

An observed effect so large that it would rarely occur by chance

Blocking

A form of control. Grouping similar experimental units together before splitting them evenly among the treatments.

Matched Pairs Design

A common type of randomized block design for comparing 2 treatments. Sometimes each subject receives both treatments in a random order. Sometimes subjects are matched up in pairs as closely as possible and each subject in the pair receives one of the treatments.

Census

selects all items/subjects in a population.

Sample Survey

a study that uses an organized plan to choose a sample that represents some specific population

Sampling Design

method used to choose the sample from the population

IRB

A review boards that oversees all studies and protects the saftey and well-being of all subjects

Informed consent

All subjects must be explicitly told before data is collected

Confidential Data

All data must be kept secret