This ratio is also known as the inverse Mills ratio (or lambda, X), whose mean values are a proxy variable for the existence of labour-market entry barriers. The sum of the estimated probabilities as a proportion of the total working-age population gives the net participation rate, estimated at the aggregate level. The elasticity is the ratio of the percentage change in the wage over the change in this rate. These variables can then be used to construct three growth models using the GDP growth rate as a dependent variable and including, alternatively, the variables for labour-market feminization and hours of work.