  The American Community Survey (ACS) provides one-year (1y), three-year (3y)
and five-year (5y) multi-year estimates (MYEs) of various demographic and
economic variables for each "community", although the 1y and 3y may not be
available for communities with a small population. These survey estimates are
not truly measuring the same quantities, since they each cover different time
spans. Using some simplistic models, we demonstrate that comparing different
period-length MYEs results in spurious conclusions about trend movements. A
simple method utilizing weighted averages is presented that reduces the bias
inherent in comparing trends of different MYEs. These weighted averages are
nonparametric, require only a short span of data, and are designed to preserve
polynomial characteristics of the time series that are relevant for trends. The
basic method, which only requires polynomial algebra, is outlined and applied
to ACS data. In some cases there is an improvement to comparability, although a
final verdict must await additional ACS data. We draw the conclusion that MYE
data is not comparable across different periods.
