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Gravity
Key Concepts:
Terms in this set (35)
Observational study
Looks at relationships between two variables, but does not influence the response. Used to make guesses or inferences about the population.
Population
The entire group about which we want information.
Sample
The subset of the population from which we collect information.
Convenience sample
Only members of the population who are easily accessible are selected. Unlikely that the sample actually represents the population.
Bias
Certain outcomes are favored.
Voluntary response sample
People choose themselves to be in the sample. Often people with strong opinions respond.
Simple random sample
Individuals are chosen in a way that every set of the same size has an equal chance to be in the sample.
Stratified random sample
The population is divided into groups where members are alike, then a random sample is taken from each group.
Cluster sample
The population is divided into groups that resemble the population. Then a random sample of groups is selected— each member of the chosen group is included in the sample.
Inference
Drawing conclusions about a population based on a sample.
Sampling variability
Sample results vary from sample to sample.
Margin of error
How far off our sample results are from the truth about the population (includes both directions)
Sampling error
Errors that have to do with choosing the sample.
Undercoverage
Some groups in the population are left out of the process. The same results may differ from the truth about the population.
Non-sampling errors
Errors that occur after the sample has been selected.
Nonresponse
An individual chosen for the sample can't be contacted or refused to participate.
Experiment
Imposes a treatment to influence the response.
Explanatory variable/factor
The variable being manipulated
Level
The values of a factor.
Treatment
The combination of factors and levels.
Response variable
The value being measured.
Lurking variable
A variable not included in a study that may confound the results.
Confounding
When variables that can't be distinguished from each other both influence the response.
Experimental units
Objects on which treatments are imposed.
Subjects
People on which treatments are imposed.
Random assignment
Experimental units are assigned to treatments using some type of chance process.
Completely randomized design
Every experimental unit has the same chance to be in each group.
Control group
A non-treatment group that provides a baseline for comparing the effects of the treatments. Not always necessary.
Replication
Using enough experimental units in each group so that any differences in the effects of the treatments can be distinguished from chance differences between groups.
Placebo effect
Experimental results are caused by expectations alone.
Single blind
One party knows, the other doesn't
Double blind
neither the subject nor those who interact with them and measure the response variable know which treatment a subject received.
Statistically significant
An observed effect so large that it would rarely occur by chance.
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.
Matched pairs design
a type of block design for comparing two treatments in which similar experimental units are paired (the treatments are randomly divided within each pair) or individuals are compared to themselves (each individual experiences both treatments, usually in random order).
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