Economists istinguish between four major types of unemployment, i.e., cyclical, frictional, structural and classical. (Another distinction, not discussed here, is between voluntary and involuntary unemployment.) Real-world unemployment may combine different types, while all four might exist at one time. The magnitude of each of these is difficult to measure, partly because they overlap and are thus hard to separate from each other. All but cyclical unemployment can be seen as existing at full employment, the level of employment and unemployment that represents the inflation barrier to demand-side growth.