Frequently Asked Questions
Index Methodology
ATA has been producing the tonnage index since the early 1970s. The index starts in January 1973 and includes both a not seasonally adjusted (NSA) and a seasonally adjusted (SA) series. The index is set to 2000 equaling 100.0. Each month, ATA asks its membership the amount of tonnage each carrier hauled, including all types of freight. The indexes are then calculated based on those responses.
The NSA index is assembled by adding up all the monthly tonnage data reported by the survey participants for the two latest months. A monthly percent change is calculated and then applied to the index number for the n-1 month (that is, a 5 percent rise between March and April tonnage reported means the index level for March is increased by 5 percent to calculate the April level.)
The SA index is obtained by applying statistical seasonal coefficients to the unadjusted index. Those coefficients are built from a statistical process that weighs in past fluctuations in the NSA index and attempts to correct for pronounced changes that occur repeatedly due to seasonal phenomena. The goal is to remove the effect of these phenomena and to obtain a better picture of the ongoing business trend. (We use software produced for the Bureau of Labor Statistics in adjusting many of their data series, including the consumer price index.)
As a policy to protect our participants, the sample size is not distributed and remains confidential. Even though the exact sample composition cannot be published either, the sample does contain a fairly proportional number of carriers to each segment of the industry, including LTL and TL (specialized, refrigerated, flatbed, dry van, and bulk/tank.).