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Bias
A systematic error that tends to cause the observations to deviate in the same direction from the truth about the population whenever a sample or experiment is repeated.
Confounding
Two variables are confounded when their effects on the outcome of a study cannot be distinguished from one another.
Control group
A group of experimental subjects who are given a standard treatment or no treatment (such as a placebo).
Convenience sample
A sample that consists of the individuals who are most easily available, such as people passing by in the street; A convenience sample is usually biased.
Double-blind experiment
An experiment in which neither the experimental subjects nor the persons who interact with them know which treatment each subject received.
Experiment
A study in which treatments are applied to people, animals, or things in order to observe the effect of the treatment.
Histogram
A graph that displays how often various outcomes occur by means of bars; The height of each bar is the number of times an outcome or group of outcomes occurred in the data.
Margin of error
As announced by most national polls, the margin of error says how close to the truth about the population the sample result would fall in 95% of all samples drawn by the method used to draw this one sample.
Observational study
A study (such as a sample survey) that observes individuals and measures variables of interest but does not attempt to influence the responses.
Placebo effect
The effect of a dummy treatment (such as an inert pill in a medical experiment) on the response of subjects.
Population
The entire group of people or things that we want information about.
Randomized comparative experiment
An experiment to compare two or more treatments in which people, animals, or things are assigned to treatments by chance.
Sample
A part of the population that is actually observed and used to draw conclusions, or inferences, about the entire population.
Simple random sample
A sample chosen by chance, so that every possible sample of the same size has an equal chance to be the one selected.
Statistical significance
An observed effect is statistically significant if it is so large that it is unlikely to occur "just by chance" in the absence of a real effect in the population from which the data were drawn.
Table of random digits
A table whose entries are the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 in a completely random order; That is, each entry is equally likely to be any of the 10 digits and no entry gives information about any other entry.
Voluntary response sample
A sample that chooses itself by responding to a general invitation to write or call with their opinions; Such a sample is usually strongly biased.