We can also use log/ln on both sides of the regression. In that case, changes in both variables are percentage changes. A classic example is a demand function where quantity demanded is on the left-hand side and the price is on the right. If both variables are in log, then the coefficient on price would be interpreted as price elasticity of demand. Interpret the coefficient of -0.2 in the hypothetical demand function below:

log Q = 10 - 0.2log P + ..........

or

ln Q = 10 - 0.2ln P + ..........