A Greek letter, written as ∑ in upper case, or σ in lower case. This is used in economics in various ways. Upper case ∑ is used to denote the sum of the items following it; thus 0198607679.sigma.1.jpg Here the subscript 1 and superscript N after ∑ denote the limits of the values i should take. In cases where the limits of the summation are obvious, σ_i may be used. Lower case σ is used to denote the standard deviation, a widely used measure of dispersion, defined as the square root of the mean sum of squares of the distances between each x_i and their mean μ_x. Thus 0198607679.sigma.2.jpg (this is true if μ_x is already known; if μ_x is itself being calculated from the same data set, 0198607679.sigma.3.jpg gives an unbiased estimate). The symbols ∑ and σ are also both frequently used in economics in other senses defined ad hoc.

• directorates, interlocking
• direct tax
• dirigisme
• dirty floating
• discomfort index
• discount
• discounted cash flow
• discounted present value
• discount house
• discounting the future
• discount rate
• discount window
• discouraged worker
• discrete distribution
• discrete time
• discrete variable
• discretionary policy
• discretionary spending
• discriminating monopoly
• discrimination
• disease, Dutch
• diseconomies of scale
• diseconomy, external
• disembodied technical progress
• disequilibrium