In the context of labour economics, inequality is usually referring to the unequal distribution of earning between households. Inequality is commonly measured by economists using the Gini coefficient. This coefficient does not have a concrete meaning but is more used as a way to compare inequality across regions. The higher the Gini coefficient is calculated to be the larger inequality exists in a region. Over time, inequality has, on average, been increasing. This is due to numerous factors including labour supply and demand shifts as well as institutional changes in the labour market. On the shifts in labour supply and demand, factors include demand for skilled workers going up more than the supply of skilled workers and relative to unskilled workers as well as technological changes that increase productivity; all of these things cause wages to go up for skilled labour while unskilled worker wages stay the same or decline. As for the institutional changes, a decrease in union power and a declining real minimum wage, which both reduce unskilled workers wages, and tax cuts for the wealthy all increase the inequality gap between groups of earners.