it lasts longer. As with frictional unemployment, simple demand-side stimulus will not work to abolish this type of unemployment easily. Structural unemployment may also be encouraged to rise by persistent cyclical unemployment: if an economy suffers from longlasting low aggregate demand, it means that many of the unemployed become disheartened, and their skills (including job-searching skills) become "rusty" and obsolete. Problems with debt may lead to homelessness and a fall into the vicious circle of poverty. That means that they may not fit the job vacancies that are created when the economy recovers. The implication is that sustained high demand may lower structural unemployment. This theory of persistence in structural unemployment has been referred to as an example of path dependence or "hysteresis." Much technological unemployment, caused by the replacement of workers by machines might be counted as structural unemployment. Alternatively, technological unemployment might refer to the way in which steady increases in labour productivity mean that fewer workers are needed to produce the same level of output every year. The fact that aggregate demand can be raised to deal with the problem suggests that the problem is instead one of cyclical unemployment. As indicated by Okun's law, the demand side must grow sufficiently quickly to absorb not only the growing labour force but also the workers who are made redundant by the increased labour productivity. Seasonal unemployment may be seen as a kind of structural unemployment since it is linked to certain kinds of jobs (construction and migratory farm work). The most-cited official unemployment measures erase this kind of unemployment from the statistics using "seasonal adjustment" techniques. That results in substantial and permanent structural unemployment. Frictional unemployment Frictional unemployment is the time period between jobs in which a worker searches for or transitions from one job to another. It is sometimes called search unemployment and can be voluntary, based on the circumstances of the unemployed individual. Frictional unemployment exists because both jobs and workers are heterogeneous, and a mismatch can result between the characteristics of supply and demand. Such a mismatch can be related to skills, payment, work-time, location, seasonal industries, attitude, taste, and a multitude of other factors. New entrants (such as graduating students) and re-entrants (such as former homemakers) can also suffer a spell of frictional unemployment. Workers and employers accept a certain level of imperfection, risk or compromise, but usually not right away. They will invest