Definition of terms
At-risk rates measure the rate of disease/outcome in the population at risk for that disease/outcome. For asthma outcomes, such as asthma hospitalizations, the population at risk used in the rate denominator is persons with asthma in the demographic group of interest. For national estimates, the estimate used for the population of persons with asthma is the number of people with current asthma obtained from the National Health Interview Survey.
Risk-based rates account for prevalence differences over time and between subgroups and can be used to directly compare the risk of an outcome (e.g., hospitalizations or deaths) among subgroups due to factors other than a difference in current asthma prevalence.
Population-based rates are used to measure the risk of disease/outcome in a given population overall without regard to disease prevalence. The population used in the denominator is the total population of the demographic group of interest. For national estimates, U.S Census population estimates for the resident population can be used. http://www.census.gov/popest/External
Commonly used rate denominators
U.S. Census population estimates (http://www.census.gov/popest/External)
The census of population (decennial census) has been held in the United States every 10 years since 1790. Since 1930, it has enumerated the resident population as of April 1 of the census year. Data on sex, race, Hispanic origin, age, and marital status are collected from 100% of the enumerated population.
Post-censal population estimates are estimates made for the years following a census, before the next census has been taken. Post-censal population estimates are derived annually by updating the resident population enumerated in the decennial census using a components-of-population change approach. Each annual series includes estimates for the current data year and revised estimates for the earlier years in the decade. The following formula is used to derive national estimates for a given year from those for the previous year, starting with the decennial census enumerated resident population as the base:
Resident population estimate + births to U.S. resident women – deaths to U.S. residents + net international migration.
Inter-censal population estimates are estimates made for the years between two decennial censuses and are produced once the census at the end of the decade has been completed. They replace the post-censal estimates produced prior to the completion of the census at the end of the decade. Inter-censal estimates are more accurate than post-censal estimates because they are based on both the census at the beginning and the census at the end of the decade. They are derived by adjusting the final post-censal estimates for the decade to correct for the error of closure (the difference between the estimated population at the end of the decade and the census count for that date). The patterns of population change observed over the decade are preserved.
Count of persons with current asthma
The number of persons with current asthma on a national level can be obtained from the National Health Interview Survey (NHIS). Current asthma is based on a yes response to both of two questions:
- “Has a doctor or other health professional EVER told you that [child’s name] had asthma?” AND
- “Does [child’s name] still have asthma?”
To match estimates in CDC publications, records with responses to these questions including “don’t know,” “not applicable” and “refused” should be excluded. As a result, the total estimated count will represent a slight undercount given that the above categories cannot be assigned an asthma status.
Race and Hispanic Origin (Ethnicity)
Ascertainment of race and Hispanic Origin has changed over time in national health surveys. For example, in the National Health Interview Survey, race was recorded solely on the basis of the interviewer’s observation, and no information was recorded about Hispanic origin through 1975. In 1977, the Office of Management and Budget (OMB) established Directive 15 to establish standards to improve information on race and ethnicity (Office of Management and Budget. Standards and guidelines for federal statistics: race and ethnic standards for federal statistics and administrative reporting. Federal Register, 1977; Circular No A48, Section 7H, Exhibit F.). In 1997, the OMB released revisions to the original Directive on which national health data collection efforts are based (Office of Management and Budget. Revisions to the standards for the classification of federal data on race and ethnicity. Federal Register, 1997; 62(210): 58782-58790). The National Health Interview Survey has detailed documentation on the data collection, data editing, imputation and release of race/ethnicity data: https://www.cdc.gov/nchs/nhis/rhoi.htm.
The health insurance standards have been adapted from the methods used by the National Health Interview Survey (NHIS). The Health Insurance section of the NHIS Family Core (FHI) has a full range of data items addressing health insurance. A family respondent answers these questions about all family members. However, individual members present can also respond to the questions as well. Although the questions are asked on the Family Core component of the questionnaire, health insurance coverage status is collected for each family member. The flow of the questions pertaining to health insurance programs in this section has been similar since 1997. The FHI section begins by asking an overall question of whether anyone in the family has health insurance coverage. If there is a positive response to this initial overall question, then the types of coverage are collected for each family member. The types include “no coverage of any type,” in case one or more family members are uninsured but others are not. The questions about types of coverage are followed by detailed questions about each coverage type collected on a person basis, with an exception of detailed questions about private plans which are asked by plan, for up to four plans per family. If there is a negative response to the initial overall question, then all family members are marked as not having health insurance coverage and the appropriate follow-up questions for those who lack coverage are asked. The FHI section ends with a series of family-level questions about out-of-pocket expenses, flexible spending accounts, and problems paying medical bills.
A feature that distinguishes the NHIS estimates of health insurance coverage from other survey-based estimates is the use of responses to follow-up questions to evaluate the reliability of the reported health insurance coverage and to adjudicate conflicting information. For many survey respondents, health insurance is a complex topic, and some inconsistencies in survey response are expected. If the follow-up questions clearly suggest that the original responses were incorrect, the original responses are edited. As a result, a portion of the sample is reassigned to a different type of coverage or reclassified from insured to uninsured (or vice versa). On the data file, the recodes are: MEDICARE, MEDICAID, PRIVATE, SCHIP, IHS, MILCARE, OTHPUB, OTHGOV, SINGLE, and NOTCOV. NOTCOV reflects the definition of non-coverage as used in Health, United States (in which persons with only Indian Health Service coverage are considered uninsured). Analysts are strongly advised to use the recodes MEDICARE, MEDICAID, PRIVATE, SCHIP, IHS, MILCARE, OTHPUB, OTHGOV, and SINGLE for estimates of types of health care coverage and NOTCOV to derive estimates of un-insurance. For more detail, please see: https://www.cdc.gov/nchs/nhis/health_insurance/hi_eval.htm.
The U.S. Census states that “income is the gauge many use to determine the well-being of the U.S. population.” However, income and poverty are complex measurements. The U.S. Census collects data on income using several household surveys, and estimates vary from the family level to more broad measures of mean income for certain geographic areas. To be comparable measures used in national surveys, measures of income and poverty should use similar methods outlined by the U.S. Census and federal statistical agencies. Two resources for more in-depth information are:
- U.S. Census “Income & Poverty”: https://www.census.gov/topics/income-poverty.htmlExternal
- Health US “Appendix II: Family Income”: https://www.cdc.gov/nchs/hus/index.htm
Four professional medical associations (American Academy of Family Physicians, American Academy of Pediatrics, American College of Physicians, American Osteopathic Association) issued joint principles of the Patient-Centered Medical Home as an approach to providing comprehensive primary health care in 2007 (http://www.aafp.org/dam/AAFP/documents/practice_management/pcmh/initiatives/PCMHJoint.pdf Cdc-pdf[PDF – 269 KB]External). The underlying goals of the medical home concept—including accessibility, continuity, comprehensiveness, family-centered, care coordination, and cultural competence—align closely with goals for high quality asthma care, management and education that are recommended in the NAEPP Guidelines (http://www.nhlbi.nih.gov/health-pro/guidelines/current/asthma-guidelines/full-reportExternal).