ch 4

population

population

a group of individuals we want info about

census

collects data from every individual in the population

sample

subset of individuals selected from the larger population from which we collect data

sample survey

a study that collects data from a sample to learn about the population from which the sample was selected

bias

is shown if a study is likely to underestimate or overestimate the value you want to know

convenience sampling

selects individuals from the population who are easy to reach

voluntary response sampling

allows people to choose to be in the sample

random sampling

selects individuals using a chance process to determine which members of a population are included in the sample

single random sample (SRS)

a sample of size N is chosen in a way that every group of N individuals in the population has an equal chance to be in the sample

sampling without replacement

an individual from a population can be selected only once

sampling with replacement

an individual from a population can be selected more than once

strata

groups of individuals in a population who share characteristics thought to be associated with the variables being measured in a study

stratified random sampling

selects a sample by choosing a single random sample (SRS) from each stratum (groups of similar people) and combining the SRSs into one overall sample (a ring around the stadium)

cluster

a group in the population that are near each other

cluster sampling (a type of convenience sampling)

selects a sample by randomly choosing clusters and including all members of the selected clusters in the sample (one section from each price point)

systematic random sampling

selects a sample from an ordered arrangement of the population by randomly selecting one of the first K individuals and choosing every Kth individual thereafter

undercoverage bias

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

response bias

when there is a systemic pattern of inaccurate answers to a survey question

nonresponse bias

when an individual chosen for the sample can't be contacted or refuses to participate

observational study

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

experiment

deliberately IMPOSES some treatment on individuals to measure their responses

explanatory variable

helps explain or predict changes in a response variable (what theyre DOING to the subject)

response variable

measures an outcome of a study (the EFFECT of what theyre doing)

confounding

occurs when two variables are associated in such a way that their effects on a response variable cannot be distinguished from each other (too similar)

placebo

a treatment that has no active ingredient

placebo effect

when some subjects in an experiment respond favorably to an inactive treatment

treatment

a specific condition applied to the individuals in an experiment

experimental unit (EU)

the object the treatment is assigned (subjects when human)

factor

an explanatory variable that is manipulated and may cause a change in the response variable

levels

different values of a factor

control group

provides a baseline for comparing the effects of other treatments (often given a placebo)

single-blind

when EITHER the subjects or those who interact with them know which treatment a subject recieved

double-blind

when NEITHER subjects nor the people who interact with them know which treatment a subject recieved

comparison (part of CRCR)

comparing two treatments

random assignment (part of CRCR)

EU are assigned to treatments by CHANCE

control (part of CRCR)

keeping all other variables constant

replication (part of CRCR)

giving each treatment to enough EUs so that a difference in effects can be distinguished from chance variation

completely randomized design

when EUs are assigned to treatments at random

block

a group of experimental units that are known to be similar in some way that will affect results

randomized block design

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

matched pair design

a design to compare 2 treatments using blocks with 2 EUs each.

type 1 of matched pair design

the two treatments are randomly assigned WITHIN the EUs

type 2 of matched pair design

each EU receives BOTH treatments in random order

sampling variability

refers to the fact that different random samples of the same size from the same population produce different samples

statistically significant

when the results are to unusual to be explained by chance (1-5%)

inference

can be made when samples are randomly selected