undeclared paid occupations may try to get unemployment benefits in addition to the money that they earn from their work. However, in the United States, Canada, Mexico, Australia, Japan, and the European Union, unemployment is measured using a sample survey (akin to a Gallup poll). According to the BLS, a number of Eastern European nations have instituted labour force surveys as well. The sample survey has its own problems because the total number of workers in the economy is calculated based on a sample, rather than a census. It is possible to be neither employed nor unemployed by ILO definitions by being outside of the "labour force." Such people have no job and are not looking for one. Many of them go to school or are retired. Family responsibilities keep others out of the labour force. Still others have a physical or mental disability that prevents them from participating in the labour force. Some people simply elect not to work and prefer to be dependent on others for sustenance. Typically, employment and the labour force include only work that is done for monetary gain. Hence, a homemaker is neither part of the labour force nor unemployed. Also, full-time students and prisoners are considered to be neither part of the labour force nor unemployed. The number of prisoners can be important. In 1999, economists Lawrence F. Katz and Alan B. Krueger estimated that increased incarceration lowered measured unemployment in the United States by 0.17% between 1985 and the late 1990s. In particular, as of 2005, roughly 0.7% of the US population is incarcerated (1.5% of the available working population). Additionally, children, the elderly, and some individuals with disabilities are typically not counted as part of the labour force and so are not included in the unemployment statistics. However, some elderly and many disabled individuals are active in the labour market. In the early stages of an economic boom, unemployment often rises. That is because people join the labour market (give up studying, start a job hunt, etc.) as a result of the improving job market, but until they have actually found a position, they are counted as unemployed. Similarly, during a recession, the increase in the unemployment rate is moderated by people leaving the labour force or being otherwise discounted from the labour force, such as with the self-employed. For the fourth quarter of 2004, according to OECD (Employment Outlook 2005