Undercoverage – Undercoverage occurs when some groups in the population are left out of the process of choosing the sample.

Matched Pair Design – Matched pairs are a common form of blocking for comparing just two treatments.

Stratified Random Sample – To select a stratified random sample, first divide the population into groups of similar individuals, called strata.  Then choose a separate simple random sample in each stratum and combine theses simple random samples to form a full sample.

Bias – The design of a study is biased if it systematically favors certain outcomes.

Treatment – A specific experimental condition applied to the units is called a treatment.

Placebo – A placebo is a dummy treatment.

Control – The group of patients who received a sham treatment is called a control group.

Statistically Significant – An observed effect so large that it would rarely occur by chance is called statistically significant.

Completely Randomized Design – When all experimental units are allocated at random among all treatments, the experimental design is completely randomized.

Blind Experiment – In a blind experiment, the subjects do not know which treatment they received.

Double-Blind Experiment – In a double-blind experiment, neither the subjects nor the people who have contact with them know which treatment a subject received.

Block Design – A block is a group of experimental units or subjects that are known before the experiment to be similar in some way that is expected to affect the response to the treatments.  In a block design, the random assignment of units to treatments is carried out separately within each block.

Confounding – Two variable are said to be confounded when their effects on a response variable cannot be distinguished from each other.

Census – A census attempts to contact every individual in the entire population.