How to Calculate Trends
In 1990, only certain manufacturing companies were required to report use of toxic materials. Starting in 1991, both certain manufacturing and certain non-manufacturing companies were required to report. Reporting progress for companies that started reporting in different years is best done by reporting progress only over the years reported in common.
Suppose, for example, that there are only two companies in a town, XYZ Corp and ABC Corp. The quantities reported might look something like this:
|
|
1990 |
1991 |
1992 |
1993 |
1994 |
1995 |
1996 |
1997 |
|
XYZ Corp |
50,000 |
60,000 |
70,000 |
80,000 |
70,000 |
50,000 |
60,000 |
70,000 |
|
ABC Corp |
0 |
110,000 |
110,000 |
100,000 |
100,000 |
120,000 |
130,000 |
130,000 |
At first glance, it would seem that the total chemical use in the town might be:
|
|
1990 |
1991 |
1992 |
1993 |
1994 |
1995 |
1996 |
1997 |
|
Total |
50,000 |
170,000 |
180,000 |
180,000 |
170,000 |
170,000 |
190,000 |
200,000 |
However, a look at the SIC codes for these companies would show that
XYZ Corp is a manufacturing company that had to start reporting in
1990, but ABC Corp is a distributor and didn't have to start reporting
until 1991. As a result, there is no way to tell from the TURA data how
much ABC used in 1990 and reporting the total as 50,000 lbs is most
likely an underestimate. In addition, because the 1990 data only
includes the XYZ's reported use, the increase from 50,000 in 1990 to
160,000 in 1991 is most likely an overstatement of the change, because
the 50,000 is only XYZ's amount and the 160,000 includes both
companies' amounts.
Given
this problem with changing reporting requirements, one way to look at
trends is to report the quantities for all companies that were required
to report in common years. For this example, it is possible to say that
the use of toluene in the town increased from 160,000 in 1991 to
200,000 in 1997. In this case, a report of use for the company might
look like this:
|
|
1990 |
1991 |
1992 |
1993 |
1994 |
1995 |
1996 |
1997 |
|
Total |
N/A |
170,000 |
180,000 |
180,000 |
170,000 |
170,000 |
190,000 |
200,000 |
** A change in reporting requirements makes it impossible to report
total use for this year for all companies in the town on a basis
consistent with the other years reported.
This page updated Friday September 23 2005