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3.3 Smoothed Rate Calculations
Geospatial smoothing can be applied to national, regional, or state maps showing county-level detail. When a county-level map indicates many unstable (or undefined) rates, geographic patterns can sometimes be clarified by displaying smoothed rates. For a given county, the smoothed rate is calculated by using the data for that county plus the data from counties that border it. The result is a rate that represents the county “neighborhood”. The neighborhood for a county includes any counties that meet it at just a corner point and any counties that share just an administrative boundary with it (for example, a boundary in a body of water). It also includes any neighboring counties across state lines. To obtain the technical report providing the details of the smoothing procedure used in WISQARS Fatal Injury Mapping, please contact CDC-INFO.
The typical county neighborhood in the U.S. includes six to eight counties, but some county neighborhoods are much smaller or much larger. A few counties are completely isolated, having no immediately neighboring counties. For such counties, smoothed rates will be identical to unsmoothed rates.
In some instances, the counties belonging to a county neighborhood may contribute different numbers of years of data to a smoothed rate calculation. Such instances are usually due to geographic changes that occurred during the years covered by a reporting period (for example, an entirely new county might be formed in the middle of a reporting period). Under such circumstances a bias correction is applied to the smoothed rate calculations in order to avoid giving counties contributing fewer years of data undue influence. To obtain the technical addendum describing this bias correction, please contact CDC-INFO.
Geospatial smoothing can be applied to both crude and age-adjusted rates. When smoothing is combined with age adjustment, the smoothing step is carried out first (within each 5-year age group), followed by the direct method of age adjustment.