The Health Professions Resource Center (HPRC) receives data files each year from each of the health professions licensing boards. Some boards supply only a mailing (residence) address while others provide both a mailing and a practice address. HPRC prefers to report data by county of practice when possible, as this provides more accurate information as to where providers are actually practicing (many providers practice in a different county from the one in which they live). To determine in which county a health provider resides or practices - and therefore which county should be assigned to the data record - HPRC has, in the past, used a file which contains all of the zip codes in Texas, and the county in which that zip code is located. However, there are many zip codes in Texas that cross county lines. Only one county name can be associated with each zip code in this method; therefore, some records with one of these zip codes get coded to the wrong counties. A further complication is that sometimes the zip code is missing while the street address and city name are available. HPRC would assign a county name based on the city name, but just as with zip codes, there are many cities in Texas that are in more than one county (several are in more than two). Therefore, mistakes are made using this method as well.
In an effort to increase the accuracy of the information reported by HPRC, in 2008, the GIS section of the Center for Health Statistics used geocoding software to assign county names to each record. This method uses the entire address and not just the zip code, which increases the accuracy of the records with zip codes and/or city names in more than one county, and also helps to correct errors made when a zip code is entered incorrectly in the database (just transposing two numbers could move a record to the other side of the state).
The most obvious result of this change in methodology is that some records that were coded to the central county of a metropolitan area in the past are now getting coded to surrounding counties. For example, some records that were coded to Harris County in the past - either because of the zip code, or because the city name was Houston - are now being coded to Fort Bend or Montgomery Counties. Some records that were coded to Dallas County in the past are now being coded to Collin or Denton Counties. In some cases, the number of records changing counties can be several hundred, for the largest counties.
Therefore, some counties will show a large increase or decrease in the numbers, compared to 2007 and past years. HPRC intends to continue geocoding in the future, so these discrepancies should disappear eventually, and, the current and future data should be more accurate than that of past years. However, some errors probably still remain, as some records do not provide complete addresses; if HPRC is not given a full street address and zip code, for example, and the only information provided is the city name of Dallas, that record will continue to be coded to Dallas County, even though the City of Dallas spans Dallas, Denton, Collin, Kaufman, and Rockwall Counties. Complete accuracy in assigning a county name, even with GIS software, can only be done if the board provides a full street address, city name, and correct zip code for each record.