| Time Dimension: presented versus 1991 ZIP | | | | analysis unless you have some way of keeping |
| Codes in one ZIP and the other side in a | | | | track of all the changes over time. |
| United States America ZIP Code. The block to | | | | Otherwise, you may wind up concluding that |
| ZIP equivalency file organized by the vendors | | | | there has been a dramatic downward trend in |
| each chunk was assign to one and only one | | | | the population of 63301 since 1980, when in |
| survey building block. | | | | fact just the opposite is true. |
| | | | |
| An important object to keep in mind about ZIP | | | | What the world really needs to deal with ZIP |
| code finder is that they change over time. In | | | | code natural features properly is a large |
| some cases these change can be quite amazing, | | | | geographic equivalency organizer linking ZIP |
| but more frequently they are small and | | | | code finder to other relevant geographies |
| subtle. When a ZIP code changes its | | | | with a time aspect. Instead, what we have is |
| definition it does not change its name like a | | | | such an equivalency file that relates ZIP |
| census zone. The ZIP code that was called | | | | codes to geographic entities primarily used |
| '63301' in St. Charles County, Mo in 1985 has | | | | for taking the 1990 census. This file uses |
| since been broken into first two and now | | | | ZIP codes as they were defined around July of |
| three ZIP codes. These new codes were not | | | | 1991. What we are referring to here is the |
| called 63301.01, 63301.02 and 63301.03; they | | | | ZIP CODE FINDER DATABASE and the |
| were called 63301, 63303 and 63304. So what | | | | corresponding Braintech web application for |
| is referred to as 63301 today represent about | | | | programming script which we'll be discussion |
| a third of the area that it referred to in | | | | about in more detail is visit you site. For |
| 1985. The new code 63303 did not exist 12 | | | | now, what we want to emphasize is that when |
| years ago and it has already changed its | | | | we talk about ZIP codes we really need to |
| definition so that it now represents about | | | | keep a time reference in mind. Just as when |
| partially of the area it included when it was | | | | you work with census tracts you need to know |
| to begin with created. What this means, of | | | | whether you mean 1980 or 1990 tracts, or when |
| course, is that ZIP codes are really terrible | | | | you are talking about the countries of Europe |
| units for doing any kind of time-series | | | | -- time is an important dimension. |