Arithmetic Mean: The sum of all scores (Xi) divided by their common number (N).

Array: In the most general sense, an explicit display of a set of observations. Usually, the term denotes some special arrangement of the observations; i.e., in order of magnitude. A frequency array is an array of frequencies according to variable values, that is to say, a frequency distribution.

Box-and-Whisker Plot: A box from hinge to hinge, barred at the median, with "whiskers" to
(a) the extremes
(b) the innermost identified values
(c) the adjacent values.

Categorical Distribution: A distribution is said to be categorical if the data are discrete or presented in class intervals.

Category: A homogeneous class or group of a population of objects or measurements. The category may be styled after one of the finite characteristics of the population or according to the limits of measurement for which observations are to be allocated to that category or frequency group. For example, people may be categorized according to sex (male or female) or age (1-5 years; 6-10; 11-15; and so on), and quantities of energy by type of fuel or type of consumer.

Central Tendency: The central tendency of a distribution refers to the place on the scale of values where a particular distribution may be centered.

Class: The total number of observations made upon a particular variable may be grouped into classes according to convenient divisions of the variable range in order to make subsequent analysis less laborious or for other reasons. A group so determined is called a class. The variable values which determine the upper and lower limits of a class are called class boundaries; the interval between them is the class interval; and the frequency falling into the class, the class frequency.

Coefficient of Variation: The standard deviation of a distribution divided by the arithmetic mean, sometimes multiplied by 100. It is used for the purpose of comparing the variabilities of frequency distributions but is sensitive to errors in the means.

Continuous Variable: A variable is said to be continuous when it may take values in a continuous range; i.e., volumes of fuel stocked or produced.

Correlation Coefficient: A correlation coefficient is a measure of the interdependence between two variables. It is usually a pure number which varies between -1.0 and +1.0, with the intermediate value of zero indicating an absence of correlation. The limiting values indicate perfect negative or positive correlation.

Dependent Variable: See Independent Variable.

Discontinuous (Discrete) Variable: A variable which can take only discrete values, i.e., cents, number of births, or number of utilities.

Factor: The word is used to denote a quantity under examination as a possible cause of variation.

Frequency: The number of occurrences of a given type of event, or the number of members of a population falling into a specified class. When the frequency is expressed as a proportion of the total number of members, it is called the relative or proportional frequency; but where no ambiguity can arise, these ratios may simply be called frequencies.

Frequency Distribution: A specification of the way in which the frequencies of members of a population are distributed according to the values of the variables which they exhibit. For observed data, the distribution is usually specified in tabular form, with some grouping for continuous variables.

Histogram: A univariate frequency diagram in which rectangles proportional to the area to the Class Frequencies are erected on contiguous sections of the horizontal axis, the width of each section representing the corresponding class interval of the variate.

Independent Variable: This term is regularly used in contradistinction to "dependent variable" in regression analysis. When a variable y is expressed as a function of variable x, the x is known as "independent variable" and y as "dependent variable."

Median: The median is that value of the variable which divides the total frequency into two halves.

Mode: The mode is the class interval in a frequency distribution with the highest value.

Open-Ended Classes: If, in a frequency distribution, the initial class interval is indeterminate at its beginning and/or the final class interval is indeterminate at its end, the distribution is said to possess "open-ended" classes.

Percentile: The set of partition values that divide the total frequency into 100 equal parts.

Range: The largest, minus the smallest in the set of variable values.

Scatter Diagram: A diagram showing the joint variation of two variables, x and y. Each member is represented by a point whose coordinates on ordinary rectangular axes are the values of the variables. A set of n observations thus provides n points on the diagram and the scatter of the points exhibits the relationship between x and y.

Skew: In a distribution, skew refers to the trailing off of frequencies toward extreme scores in one direction, away from the bulk of the cases.

Time Series: A time series is a set of ordered observations on a quantitative characteristic of an individual or collective phenomenon taken at different points of time. Although it is not essential, it is common for these points to be equidistant in time. The essential quality of the series is the order of the observations according to the time variable, as distinct from those which are not ordered at all or are ordered according to their internal properties; i.e, a set arranged in order of magnitude.

Variable: Generally, any quantity that varies. More precisely, a variable in the mathematical sense is a quantity which may take any one of a specified set of values.

Note:
The sources for these terms are:
Maurice G. Kendall and William R. Buckland, A Dictionary of Statistical Terms (London: Longman Group Ltd., 1976); Herman J. Loether and Donald G. McTavish, Descriptive and Inferential Statistics: An Introduction (Boston: Allyn and Bacon, Inc., 1976); and John W. Tukey, Exploratory Data Analysis (Reading, MA: Addison-Wesley, 1977).

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