THE EQUATION OF A LINE

Before we talk any more about lines, we need to consider the equation for a straight line. Take two variables , say X and Y. Call the variable we plot on the vertical axis Y, and the variable we plot on the horizontal axis X. Then the equation is

Y = A + B — X

The A and B in this equation are just numbers . The value A is called the intercept , and the value B is called the slope . The slope is the angle of the line and the intercept is the point where it crosses the vertical axis. If the value for the slope is positive, it tells you that as one variable increases, so does the other variable. If the slope is negative, it tells you that as one variable increases , the other decreases. If the slope is large, the line is steep, indicating that a small change in one of the variables would lead to a large change in the other variable. If the slope is small, the increase or decrease is gradual. If the slope is zero, it means that changes in the X variable have no effect on the Y variable.