mode of production, Marx argued that workers experienced alienation and estrangement through their economic identity. According to Marx, the only way to permanently eliminate unemployment would be to abolish capitalism and the system of forced competition for wages and then shift to a socialist or communist economic system. For contemporary Marxists, the existence of persistent unemployment is proof of the inability of capitalism to ensure full employment. Measurement There are also different ways national statistical agencies measure unemployment. The differences may limit the validity of international comparisons of unemployment data. To some degree, the differences remain despite national statistical agencies increasingly adopting the definition of unemployment of the International Labour Organization. To facilitate international comparisons, some organizations, such as the OECD, Eurostat, and International Labor Comparisons Program, adjust data on unemployment for comparability across countries. Though many people care about the number of unemployed individuals, economists typically focus on the unemployment rate, which corrects for the normal increase in the number of people employed caused by increases in population and increases in the labour force relative to the population. The unemployment rate is expressed as a percentage and calculated as follows: As defined by the International Labour Organization, "unemployed workers" are those who are currently not working but are willing and able to work for pay, currently available to work, and have actively searched for work. Individuals who are actively seeking job placement must make the effort to be in contact with an employer, have job interviews, contact job placement agencies, send out resumes, submit applications, respond to advertisements, or some other means of active job searching within the prior four weeks. Simply looking at advertisements and not responding will not count as actively seeking job placement. Since not all unemployment may be "open" and counted by government agencies, official statistics on unemployment may not be accurate. In the United States, for example, the unemployment rate does not take into consideration those individuals who are not actively looking for employment, such as those who are still attending college. According to the OECD, Eurostat, and the US Bureau of Labor Statistics the unemployment rate is the number of unemployed people as a percentage of the labour force. "An unemployed person is defined by Eurostat, according to the guidelines of the International Labour Organization, as: someone aged 15 to 74 (in Italy, Spain, the United Kingdom, Iceland, Norway: 16 to 74 years);