Variables like employment level, unemployment level, labour force, and unfilled vacancies are called stock variables because they measure a quantity at a point in time. They can be contrasted with flow variables which measure a quantity over a duration of time. Changes in the labour force are due to flow variables such as natural population growth, net immigration, new entrants, and retirements. Changes in unemployment depend on inflows (non-employed people starting to look for jobs and employed people who lose their jobs that are looking for new ones) and outflows (people who find new employment and people who stop looking for employment). When looking at the overall macroeconomy, several types of unemployment have been identified, which can be separated into two categories of natural and unnatural unemployment.