time. An estimated 75% are unable to work because of their illness, and about 25% are housebound or bedridden for long periods, often decades. In one review on employment status, more than half of CFS patients were on disability benefits or temporary sick leave, and less than a fifth worked full-time. In children, CFS is a major cause of school absence. According to a 1996 study by McCully et al, people with CFS report critical reductions in levels of physical activity, and according to a 2009 study, patients exhibit a reduction in the complexity of their activities. Many people with CFS also experience strongly disabling chronic pain. Symptoms can fluctuate over time, making the condition difficult to manage. Persons who feel better for a period may overextend their activities, triggering post-exertional malaise and a worsening of symptoms. Severity may also change over time, with periods of worsening, improvement or remission sometimes occurring. People with CFS have decreased quality of life according to the SF-36 questionnaire, especially in the domains of vitality, physical functioning, general health, physical role, and social functioning. However, their scores in the "role emotional" and mental health domains were not substantially lower than healthy controls. A 2015 study found that people with CFS had lower health-related quality of life than 20 other chronic conditions, including multiple sclerosis, kidney failure, and lung cancer. Cognitive functioning Cognitive dysfunction is one of the most disabling aspects of CFS due to its negative impact on occupational and social functioning. 50 to 80% of people with CFS are estimated to have serious problems with cognition. Cognitive symptoms are mainly due to deficits in attention, memory, and reaction time. Measured cognitive abilities are found to be below projected normal values and likely to affect day-to-day activities, causing increases in common mistakes, forgetting scheduled tasks, or having difficulty responding when spoken to. Simple and complex information-processing speed and functions entailing working memory over long time periods are moderately to extensively impaired. These deficits are generally consistent with the patient's perceptions. Perceptual abilities, motor speed, language, reasoning, and intelligence do not appear to be significantly altered. Patients who report poorer health status tend to also report more severe cognitive trouble, and better physical functioning is associated with less visuoperceptual difficulty and fewer language-processing complaints. Inconsistencies of subjective and observed values of cognitive dysfunction reported across multiple studies are likely caused by a number of factors.