The Time Dimension: presented versus 1991 ZIP Codes

An important object to keep in mind about ZIP code finder is that they change over time. In some cases these change can be quite amazing, but more frequently they are small and subtle. When a ZIP code changes its definition it does not change its name like a census zone. The ZIP code that was called '63301' in St. Charles County, Mo in 1985 has since been broken into first two and now three ZIP codes. These new codes were not called 63301.01, 63301.02 and 63301.03; they were called 63301, 63303 and 63304. So what is referred to as 63301 today represent about a third of the area that it referred to in 1985. The new code 63303 did not exist 12 years ago and it has already changed its definition so that it now represents about partially of the area it included when it was to begin with created. What this means, of course, is that ZIP codes are really terrible units for doing any kind of time-series analysis unless you have some way of keeping track of all the changes over time. Otherwise, you may wind up concluding that there has been a dramatic downward trend in the population of 63301 since 1980, when in fact just the opposite is true.

What the world really needs to deal with ZIP code natural features properly is a large geographic equivalency organizer linking ZIP code finder to other relevant geographies with a time aspect. Instead, what we have is such an equivalency file that relates ZIP codes to geographic entities primarily used for taking the 1990 census. This file uses ZIP codes as they were defined around July of 1991. What we are referring to here is the ZIP CODE FINDER DATABASE and the corresponding Braintech web application for programming script which we'll be discussion about in more detail is visit you site. For now, what we want to emphasize is that when we talk about ZIP codes we really need to keep a time reference in mind. Just as when you work with census tracts you need to know whether you mean 1980 or 1990 tracts, or when you are talking about the countries of Europe -- time is an important dimension.

This article write Karl Garcia will be dealing with ZIP codes as they were defined for the purpose of tabulate the 1990 survey ZIP code summary.

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