observational study
observes individuals and measures variables of interest but does not attempt to influence the responses. The purpose of an observational study is to describe some group or situation.
experiment
deliberately imposes some treatment on individuals in order to observe their responses. The purpose of an experiment is to study whether the treatment causes a change in the response
Explanatory variable
A variable thought to explain or even cause changes in another variable.
sampling design
describes exactly how to choose a sample from the population.
sample
part of the population from which we actually collect information. We use a sample to draw conclusions about the entire population.
population
statistical study is the entire group of individuals about which we want information.
convenience sample
The easiest�but not the best�sampling design just chooses individuals close at hand
voluntary response sample
lets individuals choose whether to participate or not. All write-in, call-in, and online polls are examples of voluntary response samples
probability sampling
A sample chosen by chance is called a probability sample.
Simple Random Sample
of size n consists of 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.
sample survey
an observational study that relies on a random sample drawn from the entire population.
Undercoverage
Problem with sample survey that occurs when some groups in the population are left out of the process of choosing the sample; for example, homeless individuals are left out of all government surveys that select sample households.
Nonresponse
a particularly serious source of bias in most sample surveys and occurs when a selected individual cannot be contacted or refuses to participate
response bias
The behavior of the respondent or of the interviewer in sample results results. EXAMPLE such as how many partners have you had,
comparative observational studies
case studies and cohorts
Case-Control Study
observational study, case-subjects are selected based on defined boutcomes, and a control group of subjects is selected separately to serve as a baseline with which the case group is compared. (retrospective) A random sample of individuals with a conditio
Cohort studies
subjects sharing a common demographic characteristic are enrolled and observed at regular intervals over an extended period of time. (prospective) When the study is over, individuals who have developed a condition are compared with the remaining, unaffect
Randomized Comparative Experiment
An experiment that uses both comparison of two or more treatments and chance assignment of subjects to treatments is a
matched pairs
compares just two treatments. Choose pairs of subjects that are as closely matched as possible. These can be individuals of the same sex, age, weight, and stature, for example, or they can be genetically related individuals such as twins or animals born i
Block Design
a group of individuals that are known before the experiment to be similar in some way that is expected to affect the response to the treatments. --> the random assignment of individuals to treatments is carried out separately within each block.