Question

When the price of a good falls and you buy the same quantiy of that good is it substitution effect or income effect?

Answer 1

It is neither substitution effect nor income effect, because change in price lead to change in quantity demanded of that good, due to these effects remaining other things constant.

Income effect states that with the change in price level, real income ( purchasing power) changes and which results in change in the demand of that good. If the good is normal, due to fall in price level, consumer's real income increases and he consumer more of that good.

Substitution effect states that consumer will buy cheaper good, if the price of a good falls , he will buy more of that good.

Here the quantity demanded is same, means that something other must have changed, not all other factors are kept constant.