The elasticity coefficient is a number measured using price and quantity data to verify how responsive consumers are to changes in the price of a commodity.  The elasticity coefficient may be measured in two distinct ways. Measuring responsiveness at a specific point along a demand curve.  The other method is using the mid-point of the dissimilarity in the price and the mid-point in the dissimilarity of the quantity numbers.  Because the midpoints formula cuts down on the confusion of which prices and quantities are to be used, it is the only coefficient we will use in this course.