Qualitative economics

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Qualitative economics refers to representation and analysis of information about the direction of change (+ or -) in at least one economic variable as related to change of at least one other economic variable (James Quirk, 1987, p. 1). What makes the change of a variable qualitative is that the direction (signed by a + or -) but not the magnitude of the change is specified.

Typical exercises of qualitative economics include comparative-static changes in microeconomics or macroeconomics and comparative equilibrium-growth states in a macroeconomic growth model. An example illustrating qualitative change is from macroeconomics. Let:

GDP = nominal gross domestic product, a measure of national income
M = money supply
T = total taxes.

Monetary theory hypothesizes a positive relationship between GDP the dependent variable and M the independent variable. Equivalent ways to represent such a qualitative relationship between them are as a signed functional relationship and as a signed derivative:

+ \,\!
GDP = f(M) \,\! or \,\! \frac{ df(M) }{ dM} > 0.

where the '+' indexes a positive relationship of GDP to M.

Another model of GDP hypothesizes that GDP has a negative relationship to T. This can be represented similarly to the above, with a theoretically appropriate sign change as indicated:

- \,\!
GDP = f(T) \,\! or \,\! \frac{ df(T)}{ dT} < 0.

A combined model uses both M and T as independent variables. The hypothesized relationships can be equivalently represented as signed functional relationships and signed partial derivatives (suitable for more than one independent variable):

+ \,\! - \,\!
GDP = f(M, T) \,\! or \,\! \frac{\partial f(M, T)}{\partial M} > 0, \frac{\partial f(M, T)}{\partial T} < 0.

A classic exposition of qualitative economics is Paul A. Samuelson (1947).

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