We studied elasticity in a previous post:

**The percentage change in quantity demanded, given a percentage change in price.**

In today’s post, we expand the definition of elasticity a little. That naturally makes it a little complicated, but it also enriches our understanding of it – a good bargain.

What if the price of a substitute changes? What if, that is, the price of Coke changes a little. By what percentage will the quantity demanded of Pepsi change? The measurement of such a thing is called cross price elasticity (substitute).

**The percentage change in quantity demanded, given a percentage change in the price of a substitute.**

The first definition above is therefore the definition of **own price elasticity**, while the second one is of cross price elasticity. Cross price elasticity, naturally, will be of twp types – that of complements, and that of substitutes.

There is yet a fourth type of elasticity, called income elasticity of demand. As you might imagine, it is

**The percentage change in quantity demanded, given a percentage change in income.**

Say your income in a particular month goes down by 10 percent. Is it reasonable to imagine that you will therefore cut back on your consumption of movies in a theatre, or dinners in restaurants? Unless you are a hardcore movie buff, or love eating out a lot, the answer would probably be yes. The income elasticity of demand for these goods is therefore high.

On the other hand, will you cut back your consumption of pills prescribed by your doctor? Almost definitely not, right? The income elasticity of demand for these goods is therefore low.

And that concludes our series on the basics of supply and demand!

Here’s a quick recap:

The demand (and supply) of a good depends upon:

- it’s own price
- the price of complements and substitutes
- it’s own price elasticity
- the cross price elasticities
- the income elasticity
- changing tastes and preferences
- changing incomes

As you can no doubt see, thinking about demand is fairly complex – but it is, nonetheless, rewarding. In the next post, we’ll give you a list of resources for learning more about demand and supply (as we did for the Solow model), and then begin a new topic.

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