We have seen that when both demand and supply shift, we can predict the direction of change in either equilibrium price or quantity, but not both. The table below summarizes what we have learned about the effect of a shift in both curves.

We can use our understanding of these relationships to determine the most likely causes of changing prices and quantities. For example, if we observe that in some market price is remaining relatively constant but that quantities sold are rising, we should look for changing conditions that would cause demand to increase, and other changing conditions that would lead to an increase in supply.

On the other hand, if we find quantity remaining relatively unchanged but price rising, we can look for something that would lead to a shift out in demand, and some other change that would cause supply to shift back. Similar reasoning can be applied to other possibilities.