The Centers for Disease Control and Prevention's (CDC) Environmental Public Health Tracking Network provides users access to nationally consistent health, exposure, and environmental hazard data.

The Tracking Network allows users:

• to view data in maps, tables, and charts;
• search and view metadata; and
• find information about their health and their environment.

Data are presented as measures and organized by indicator and content area. A content area is a topic within environmental public health. Content areas may focus on health, exposure, the environment, or the intersection of health and the environment.

Important Definitions

The following terms and definitions are used in the Tracking Network:

Indicator: for Tracking, an indicator is one or more items, characteristics or other things that will be assessed and that provide information about a population's health status, their environment, and other factors with the goal allowing us to monitor trends, compare situations, and better understand the link between environment and health. It is assessed through direct and indirect measures (e.g. levels of a pollutant in the environment as a measure of possible exposure) that describe health or a factor associated with health (i.e., environmental hazard, age) in a specified population. A content area may have more than one indicator.

Measure: on the Tracking Network, a measure is a summary characteristic or statistic, such as a sum, percentage, or rate. There may be several measures of a specific indicator which when considered in conjunction fully describe the indicator.

In addition to numbers or percentages, a measure can be a ratio, proportion, or rate.

Ratio: the number of events or cases that meet a set of criteria divided by the number of events or cases that meet a different set of criteria. Ratios are used to compare the occurrence of a variable in two different groups. An example is the ratio of males to females among term singleton births.

Proportion: a type of ratio which is the number of events or cases that meet a set of criteria divided by the maximum number of events or cases that could meet those criteria. In this case, the numerator is included in the denominator. Proportions are usually expressed as percentages. An example is the number of low birth weight births among all term singleton births.

Incidence Rate: the number of new cases of disease over a period of time divided by the population at risk. An example is the number of new bladder cancer cases per 100,000 persons.

Prevalence: the number of existing cases of disease at a point in time divided by the total population. An example is the number of existing cases of a birth defect per 10,000 live births.

Data Differences

Data presented on CDC's Tracking Network may differ from data that are presented on state tracking networks, state health department Web sites, and other source Web sites for the same measures. The differences may occur for many reasons, such as

• a state's use of its own population estimates.
• differences in processes for updating data.
• differences in how a measure is defined for environmental public health tracking purposes.

Consult the indicators and measures descriptions and metadata that are provided on this site for more information.

Missing Data vs. No Events

States or counties where there are no health outcome cases or no measured occurrences of an environmental hazard are labeled as "no events." States or counties with no data either did not collect data or did not report data to CDC. For example, some counties and states do not have air monitors, and some community water systems do not sample or test for each hazard every year or every reporting period. Additional data may be available through the original data source.

Privacy and Confidentiality

For the Tracking Program, data privacy means that health data will not be used for anything other than the specific public health reason for which it was shared. Confidentiality means making sure that health information is only seen by people who are authorized to have access to it.

No personal identifiers are used in creating data tables for CDC's Tracking Network. For rare health outcomes, the number of cases for a selected place, time, and group of people is often small, particularly in sparsely populated areas. In these situations, data are aggregated across place, time, and people in order to balance, as much as possible, utility and the protection of confidentiality. For example, a measure may be presented as an average over a 5-year period instead of an annual number, or a measure might only be viewed by race or age but not by race and age at the same time.

When small cell counts exist, they are suppressed (not shown). Non-zero counts that are less than 6 are suppressed for counties with a total population that is less than 100,000 persons. For cancer data, all non-zero counts that are less than 16 are suppressed, also to account for stability. To impede reverse calculation of suppressed data through multiple queries, selected non-zero counts that are greater than 6 are also suppressed.

For example, if for a given county with a total population that is less than 100,000 persons, there are 7 asthma hospitalizations among males and 3 among females, then the 3 would be suppressed. The 7 hospitalizations among males would also be suppressed, so that the county total (10) and the male total (7) could not be used to calculate the female total (3).

Stability

Rates, proportions, and percentages are checked for their stability, so that trends over time and between geographic areas or persons can be evaluated with reasonable confidence. Instability can arise from small numerators (number of cases or events) or small denominators (populations or subpopulations). Any rate or measure with a relative standard error (RSE) greater than or equal to 30% is flagged as unstable or, in the case of cancer data, suppressed. Statistical stability and confidentiality go hand in hand. The numerator required to generate an RSE of less than 30% is well above the safety cutoff level of 6. Suppression of any rate or measure due to instability also protects confidentiality.

Smoothing

One option available is to obtain geographically smoothed rates, proportions, and percentages. Smoothed measures are geographically based averages. Geographic smoothing algorithms borrow information from neighboring areas to stabilize results from sparsely populated areas. This reduces the variability in the data, allowing patterns to emerge, but it increases the bias in the estimates for each small area. Smoothed rates or measures for any single county should not be interpreted as the result for that county; instead, such rates or measures are used to identify patterns or trends across a state or group of counties.

More information about indicators, measures, and data can be found in the indicator documentation and the metadata.