Population
The entire group of individuals or instances about whom we hope to learn
Sample
a subset of the population examined in hope of learning more about the population
Sample Survey
A study that asks questions of a sample drawn from some population in the hope of learning something about the entire population
Bias
A systematic failure of a sampling method to represent the entire population; tends to over or under estimate parameters
Types of bias
voluntary/non- response bias, undercoverage, response bias
Randomization
each individual has an even chance of being selected
census
a sample consisting of the entire population
population parameter
a numerically valued attribute of a model (ex: mean income of all employed people in the country)
Statistic
values calculated for the sample data
Representative
a sample whose statistics accurately reflect the corresponding population parameters
Sampling frame
a list of individuals from whom the sample is drawn
Sampling variability
the natural tendency of randomly drawn samples to differ
stratified random sample
a sampling design in which the population is divided into several subpopulations, or strata, and random samples are then drawn from each stratum.
Strata
A homogenous section of the data (one layer of the cake)
Cluster Sample
A sampling design in which entire groups, or clusters, are chosen at random
Cluster
representative of the entire population; heterogeneous and similar to each other
Multistage Sample
Sampling schemes that combine several sampling methods
Systematic sample
selecting individuals systematically from a sampling frame
Pilot
trial run of a survey to check whether questions are clear; reduces errors due to ambiguous questions
Voluntary response bias
when individuals choose whether to participate or not; always invalid
Convenience sample
consists of individuals who are convenient to sample; not representative
Undercoverage
Biases the sample in a way that gives a part of the population less representation than it has in the overall population
Nonresponse bias
when a large fraction of those sampled fails to respond; those who do respond do not represent the entire population.
Response bias
Anything in a survey design that influences responses; ex, wording of questions
Observational Study
a study of data that has not been manipulated
Retrospective study
an observational study in which subjects are selected and then their previous conditions or behaviors are examined
Prospective study
an observational study in which subjects are followed to observe future outcomes
Experiment
A study that manipulates factor levels to create treatments, randomly assigns subjects to these treatment levels, and then compares the responses of the subject groups across treatment levels
Factor
A variable whose levels are manipulated by the experimenter. Experiments attempt to discover the effects that differences in factor levels may have on the responses of the experimental units
Response variable
A variable whose values are compared across difference treatments
Experimental units
individuals on whom the experiment is performed; called subjects or participants if they are human
Level
The specific values that the experimenter chooses for a factor
Treatment
the controlled circumstance applied to randomly assigned experimental units. the different levels of a single factor
Principals of experimental design
Control, randomization, replication, blocking
Statistically significant
when an observed difference is too large for us to believe it occurred naturally
control group
the experimental units assigned to a baseline treatment level, typically the default treatment or a placebo treatment--basis for comparison
blinding
any individual associated with an experiment who is not aware of how subjects have been allocated to treatment groups
single and double blind
blinding those who influence results (single), or that and those who evaluate the results (double)
placebo
a treatment known to have no effect; administered to maintain the same conditions within groups
placebo effect
the tendency of many human subjects to show a response when administered a placebo
blocking
when groups of experimental units are similar, you gather them into blocks: isolates the variability attributable to the differences between the blocks so we can see the affect of the treatments more clearly
matching
in a retrospective or prospective study, subjects who are similar in ways not under study are matched and compared with each other on the variables of interest
Designs
randomized and blocked
Confounding variable
when the levels of one factor are associated with the levels of another factor in such a way that their effects cannot be separated, the levels are confounded