observational study
observe individuals and measure variables without influence
experiment
deliberately impose some "treatment" to observe their responses
population
entire group of individuals
sample
part of population in order to gather info
sampling
studying a part in order to gain info
census
attempt to contact every individual in the population
bias
systematically favors certain outcome
voluntary response sample
- people who choose themselves by general appeal
- biased because people with strong opinions, especially negative, are most likely to respond
simple random sample (SRS)
N individuals from the population chosen so that every set on N individuals has an equal chance
stratified random sample
- divide the population into stratas that are similar in some way of important response
- choose separate SRS and form sample
cluster sampling
- divide the population into groups (clusters)
- randomly selected for sampling
convenience sampling
choosing individuals who are easiest to reach
multistage sampling
multiple steps (stages) of sampling; form of cluster
judgement sample
- obtained according to the discretion of someone who is familiar with relevant characteristics of the population
- based on your views
systematic sample
- picking individuals based on a system
under coverage
when some groups in the population are left out of sampling
non response
when an individual chosen for the sample cannot be contacted or don't cooperate
wording bias
the wording of the question may be loaded in some way to unduly favor one response over another
interviewer bias
how they present themselves may affect the interviewee's response
Probability sample
- ex. SRS
- a sample chosen by chance
- we must know what samples are possible and what chance, or probability, each possible sample has