of definition Some critics believe that current methods of measuring unemployment are inaccurate in terms of the impact of unemployment on people as these methods do not take into account the 1.5% of the available working population incarcerated in US prisons (who may or may not be working while they are incarcerated); those who have lost their jobs and have become discouraged over time from actively looking for work; those who are self-employed or wish to become self-employed, such as tradesmen or building contractors or information technology consultants; those who have retired before the official retirement age but would still like to work (involuntary early retirees); those on disability pensions who do not possess full health but still wish to work in occupations that suitable for their medical conditions; or those who work for payment for as little as one hour per week but would like to work full time. The last people are "involuntary part-time" workers, those who are underemployed, such as a computer programmer who is working in a retail store until he can find a permanent job, involuntary stay-at-home mothers who would prefer to work, and graduate and professional school students who are unable to find worthwhile jobs after they graduated with their bachelor's degrees. Internationally, some nations' unemployment rates are sometimes muted or appear less severe because of the number of self-employed individuals working in agriculture. Small independent farmers are often considered self-employed and so cannot be unemployed. That can impact non-industrialized economies, such as the United States and Europe in the early 19th century, since overall unemployment was approximately 3% because so many individuals were self-employed, independent farmers; however, non-agricultural unemployment was as high as 80%. Many economies industrialize and so experience increasing numbers of non-agricultural workers. For example, the United States' non-agricultural labour force increased from 20% in 1800 to 50% in 1850 and 97% in 2000. The shift away from self-employment increases the percentage of the population that is included in unemployment rates. When unemployment rates between countries or time periods are compared, it is best to consider differences in their levels of industrialization and self-employment. Additionally, the measures of employment and unemployment may be "too high." In some countries, the availability of unemployment benefits can inflate statistics by giving an incentive to register as unemployed. People who do not seek work may choose to declare themselves unemployed to get benefits; people with