Changes

Unlike price elasticity, income elasticity can be either positive or negative, depending on the good.  For a "normal" good, income elasticity is positive; for an "inferior" good, it is negative.  Inferior goods are rare: they have a decrease in demand when income by the public increases.  Usually demand for goods increase as income by the public increases, hence the name "normal good" to describe them.   

Unlike price elasticity, income elasticity can be either positive or negative, depending on the good.  For a "normal" good, income elasticity is positive; for an "inferior" good, it is negative.  Inferior goods are rare: they have a decrease in demand when income by the public increases.  Usually demand for goods increase as income by the public increases, hence the name "normal good" to describe them.   

Normal goods are necessities or luxuries, depending on their elasticity:  necessities are inelastic (elasticity less than 1), and luxuries are elastic (elasticity greater than 1).  The mistake on Question 4 of Lecture Three by your teacher was when he described "necessity" and "luxury" goods in terms of '''''price''''' elasticity rather than '''''income''''' elasticity.

Normal goods are necessities or luxuries, depending on their elasticity:  necessities are inelastic (elasticity less than 1), and luxuries are elastic (elasticity greater than 1).  Notice that "necessity" and "luxury" goods are defined in terms of '''''income''''' elasticity, not in terms of '''''price''''' elasticity.

This table is a useful study guide for income elasticity:

This table is a useful study guide for income elasticity: