Before considering specific economic examples, let us look at the issue of causality in principle. A well-known method of illustrating potential linkages between two variables, economic or otherwise, involves the

of

on a

. We position points on the graph as coordinates determined by corresponding values of two variables, in general, x and y. Thus, we might graph the heights and weights of persons in a particular village, the corresponding investment and output levels for each of the world's economies or diameter against distance from the Sun for each of the planets in the Solar System. For any particular set of data our points could appear to be scattered entirely at

(Figure la). Alternatively, they may all be

in one

of the

(Figure lb). Of particular interest to us is the case in which the observations are

in such a way that it

that a

could be drawn to connect each with the next. Suppose we had plotted data such that we could draw a straight line through all these points, the line

from the origin of the graph (Figure 1c). This picture tells us that a high degree of

exists between the two

under consideration; when x is

, then so is y, and

x values correspond to high y values.

(Fom:

Invitation to economics , by David Whynes, Martin Robertson, 1983)