In public health and in medicine, the concept of health-related quality of life refers to a person or group's perceived physical and mental health over time. Physicians have often used health-related quality of life (HRQOL) to measure the effects of chronic illness in their patients to better understand how an illness interferes with a person's day-to-day life. Similarly, public health professionals use health-related quality of life to measure the effects of numerous disorders, short- and long-term disabilities, and diseases in different populations. Tracking health-related quality of life in different populations can identify subgroups with poor physical or mental health and can help guide policies or interventions to improve their health.

Answer the set of questions called the "Healthy Days Measures" to measure health-related quality of life. These questions include the following:

Would you say that in general your health is:

• Excellent
• Very good
• Good
• Fair or Poor

Now thinking about your physical health, which includes physical illness and injury, for how many days during the past 30 days was your physical health not good?

Now thinking about your mental health, which includes stress, depression, and problems with emotions, for how many days during the past 30 days was your mental health not good?

During the past 30 days, for about how many days did poor physical or mental health keep you from doing your usual activities, such as self-care, work, or recreation?

This section includes the four core questions above, and ten additional questions about health-related quality of life. These questions ask about recent pain, depression, anxiety, sleeplessness, vitality, and the cause, duration, and severity of a current activity limitation an individual may have in his or her life.

How is the summary index of unhealthy days calculated?
Unhealthy days are an estimate of the overall number of days during the previous 30 days when the respondent felt that either his or her physical or mental health was not good. To obtain this estimate, responses to questions 2 and 3 are combined to calculate a summary index of overall unhealthy days, with a logical maximum of 30 unhealthy days. For example, a person who reports four physically unhealthy days and two mentally unhealthy days is assigned a value of six unhealthy days, and someone who reports 30 physically unhealthy days and 30 mentally unhealthy days is assigned the maximum of 30 unhealthy days.

Healthy days are the positive complementary form of unhealthy days. Healthy days estimates the number of recent days when a person's physical and mental health was good (or better) and is calculated by subtracting the number of unhealthy days from 30 days.

• Americans said they feel unhealthy (physically or mentally) about 6 days per month.
  
• Americans said they feel "healthy and full of energy" about 19 days per month.
  
• Nearly one-third of Americans say they suffer from some mental or emotional problem every month—including 10 percent
  who said their mental health was not good for 14 or more days a month.
  
• Younger American adults, aged 18–24 years, suffered the most mental health distress.
  
• Older adults suffered the most poor physical health and activity limitation.
  
• Native Americans and Alaska Natives have reported the highest levels of unhealthy days among American race/ethnicity groups.
  
• Adults with the lowest income or education reported more unhealthy days than did those with higher income or education.
  
• Americans with chronic diseases or disabilities reported high levels of unhealthy days.

Disease Specific Findings from the National Center for Chronic Disease Prevention and Health Promotion

Arthritis . . . A recent cross-sectional study of almost 10,000 adults found that adults with arthritis reported 4.6 more unhealthy days per month compared to adults without arthritis. Among adults with arthritis, the largest number of unhealthy days was experienced by women, younger persons, and persons with less than a college education.

Breast Cancer . . . Findings revealed that of almost 338,000 women surveyed, women with breast cancer reported experiencing 8.5 unhealthy days per month compared to 6.1 unhealthy days per month for women without breast cancer.

Cardiovascular Disease . . . Findings revealed that among 50,938 respondents in 21 states, persons reporting having had a heart attack, coronary heart disease, or stroke reported an average of 10 unhealthy days for the prior month compared to 5 unhealthy days reported among persons not having had one of these conditions.

Diabetes . . . Findings revealed that of more than 500,000 adults surveyed, adults with diabetes reported experiencing 9.9 unhealthy days per month compared to 5.1 unhealthy days per month for adults without diabetes.