Statistics: Sampling, Bias, and Experimental Design Key Terms

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

interviewer chooses individuals who are easiest to reach

bias

occurs when the design of a study systematically favors certain outcomes

voluntary response sample

consists of people who choose themselves by responding to a general appeal

simple random sample (SRS)

is chose so that every set of n individuals has an equal chance to be the

sample already selected

table of random digits

a long string of digits such that each entry in the table is equally likely to be any of the 10 digits 0-9 and the entries are independent of each other

stratified random sample

an SRS is chosen groups of similar individuals called strata

cluster sample and clusters

divide the population into smaller groups which mirror the characteristics of the population. Then an SRS of the clusters is chosen

strata

are similar within, but different between

clusters

are similar between but different within

inference

the process of drawing conclusions about a population on the basis of sampledata

sampling frame

the list of individuals from the sample is drawn

undercoverage

occurs when some groups in the population are left out of the sampling

process

nonresponse

occurs when a chosen individual can't be contacted or refuses to participate

response bias

a systemic pattern of incorrect responses in a sample survey

observational study

observes individuals and measures variables of interest

experiment

deliberately imposes some treatment on individuals to measure their

responses

lurking variable

a variable that is not among the explanatory or response variables in a study but that may influence the response variable

confounding

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

treatment

specific condition applied to the individuals in an experiment

experimental units

the smallest collection of individuals to which treatments are applied

subjects

human experimental units

factors

explanatory variables

random assignment

experimental units are assigned to treatments using a chance process

completely randomized design

the treatments are assigned to all the experimental units completely by

chance

control group

provides a baseline for comparing the effects of the other treatments

replication

using enough experimental units to be sure that results did not happen by

chance

placebo effect

the response to a dummy treatment

double-blind experiment

neither the subjects nor those who interact with them and measure the response variable know which treatment a subject received

single-blind experiment

the subjects are unaware of which treatment they are receiving, but the people interacting with them and measuring the response do know

statistically significant

an observed effect so large that it would rarely occur by chance is

block

a group of experimental units that are known before the experiment to be similar in some way that is expected to affect the response to the treatments

randomized block design

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

matched pairs design

blocks are created by matching pairs of similar experimental units