treatment, and the other unit receives the other treatmentMean —sum of all the values in a dataset divided by the number of values
Median —halfway through an ordered dataset, below and above which lies an equal number of data
values; 50th percentile
Mode —most common value in a distribution
Mound-shaped (bell-shaped) —distribution in which data values tend to cluster about the center of the
distribution; characteristic of a normal distribution
Mutually exclusive events —events that cannot occur simultaneously; if one occurs, the other doesn’t
Negatively associated —larger values of one variable are associated with smaller values of the other;
see association
Nonresponse bias —occurs when subjects selected for a sample do not respond
Normal curve —familiar bell-shaped density curve; symmetric about its mean; defined in terms of its
mean and standard deviation;
Normal distribution —distribution of a random variable X so that P (a < X < b ) is the area under the
normal curve between a and b
Null hypothesis —hypothesis being tested—usually a statement that there is no effect or difference
between treatments; what a researcher wants to disprove to support his/her alternative
Numerical data —see quantitative data
Observational study —when variables of interest are observed and measured but no treatment is
imposed in an attempt to influence the response
Observed values —counts of outcomes in an experiment or study; compared with expected values in a
chi-square analysis
One-sided alternative —alternative hypothesis that varies from the null in only one direction
One-sided test —used when an alternative hypothesis states that the true value is either less than or
greater than the hypothesized value but not both
Outcome —simple events in a probability experiment
Outlier —a data value that is far removed from the general pattern of the data
P (A and B) —probability that both A and B occur; P (A and B) = P (A · P (A|B)
P (A or B) —probability that either A or B occurs; P (A or B) = P (A) + P (B) – P (A and B)
P value —probability of getting a sample value at least as extreme as that obtained by chance alone
assuming the null hypothesis is true
Parameter —measure that describes a population
Percentile rank —proportion of terms in the distributions less than the value being considered
Placebo —an inactive procedure or treatment
Placebo effect —effect, often positive, attributable to the patient’s expectation that the treatment will
have an effect
Point estimate —value based on sample data that represents a likely value for a population parameter