unit 1 ap stats .2

observational study

Observes individuals and measures variable of interest but does not attempt to influence the response

retrospective study

Researchers first select the subjects and then their past events or existing conditions, to identify potential causes or outcomes of interest

prospective study

observes a group of participants over time to collect data on current exposure and track future outcomes or events

experiment

a scientific procedure undertaken to make a discovery, test a hypothesis,or demonstrate a known fact

• Used for cause and effect relationship

treatment

Specific conditions applied to the individuals in an experiment

experimental unit

Smallest collection of individuals to which treatment are applied

subject

When the units are humans

explanatory variable

Helps predict or explain changes in the response variable, manipulated by the researchers

response variable

What is measured at the end of the study

confounding variable

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

completely randomized design

experimental units are assigned to the treatments completely by chance

comparison, random assignment, control, replication

4 principles of experimental design

comparison

use a design that compares 2 or more treatments

random assignment

• Use chance to design experimental units to treatments

• Helps create equivalent groups of experimental units by balancing the effects of other variables among the treatment group

control

• Keep other variables that might affect the response the same for all groups

• Baseline for comparison (how do we know if it worked?)

replication

repeating an experiment or study under the same or similar conditions to test the validity of original findings

placebo

An inactive treatment

placebo effect

favorable response to a fake treatment

statistically significant

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

• Strong evidence that it was the treatments that caused the difference

block

An experimental design technique that groups similar experimental units into blocks to reduce variability and control the effect of a potential confounding variable

paired design

• Each subject receiving both treatments in random order

• Create blocks by matching pairs of similar experimental units. Then use chance to decide which member gets the treatment