The County Health Rankings team draws upon the most reliable and valid measures available to compile the Rankings.
One of the primary concerns when trying to estimate values for relatively small areas, like counties, is the reliability of the data used. The Rankings makes every effort to provide the most reliable data available, but all counties need to be aware that the reliability of measures varies. An easy estimate of reliability is the margin of error for a measure. Larger margins of error suggest smaller reliability.
Although the reliability of some Rankings’ measures is not ideal, when multiple measures are used to capture an underlying concept, the reliability improves. For example, each of the single measures used in morbidity may suffer from some reliability weaknesses, but taken together, their measurement of morbidity has shown great reliability.
Where possible, we provide the margins of error (95% confidence intervals) for our measure values. In many cases, the values of specific measures in different counties are not statistically different from one another; however, when combined using our model, those various measures produce the different rankings.
Treatment of missing values
In many ranked counties, some individual measures do not have a large enough sample size to report data for that measure. In these counties, we assigned the state average for any missing measure in order to calculate a rank for that category. On the map for each measure, counties with missing values will be identified with N/A (not available).
Age-adjustment of measures
Age-adjustment is a useful strategy to increase the comparability of health measures between counties. However, it can also mask the true burden of a health need in a county. Due to these considerations, only some measures in the Rankings are age-adjusted. The following Health Outcome measures are age-adjusted: premature death (YPLL), self-reported health, physically unhealthy days, and mentally unhealthy days. Preventable hospital stays and Flu Vaccinations are also age-adjusted.