When considering the income elasticity of demand principle, it is
important to understand two things:

- The equation to derive the income elasticity of demand.
- What the sign of the income elasticity of demand means.

The equation for deriving the income elasticity of demand is very
similar to the equation for the price elasticity of demand. Instead of having percent change in quantity
over percent change in price, we have percent change in quantity over percent
change in income, or

% Change in Q / % Change in I = IEoD

So if the % change in both quantity and income is positive, then
the sign of the whole equation is positive.
In this case, we would have a normal good. However, if the percent change in quantity is
negative while the percent change in income is positive, then the sign of the
whole equation would negative and we would have an inferior good.

Consider the demand for a California criminal lawyer. Generally lower income individuals need
criminal lawyers so we could assume that the income elasticity of demand
measure for a criminal lawyer would be negative. This means that as one’s income goes down,
the quantity demanded of criminal lawyers would rise.

However, if we looked at the market for criminal defense attorney in los angeles we may
see a different story. Since Los Angeles
has a lot of celebrities, it is possible that rich people would need more
lawyers for drug possession, or drunk driving, etc. In this case as income became higher, we
would see a higher quantity of criminal lawyers being demanded.

Those are the two things you need to remember when
dealing with IEoD, the equation and what the sign means.