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American Community Survey (ACS) estimates are period estimates that describe the average characteristics of population and housing over the entire period of data collection. ACS estimates give equal weight to each month in the data collection period. No month or year is given a preference. The 2007 ACS 1-year estimates are averages of the characteristics of an area over the period from January 2007 to December 2007 because this is the period of time for which sample data were collected. Similarly, the 2005 - 2007 ACS 3-year estimates are averages of the characteristics of an area over the 36 months from January 2005 to December 2007 because this 36-month period of time was the period of data collection. For example, say the 2005 - 2007 3-year estimate of the number of children in poverty for an area is 2,500. This estimate could result from many scenarios. It could be that the number of children in poverty in each of the three years was about 2,500 or that it steadily increased or decreased, for example from about 2,000 to 2,500 to 3,000, or from about 4,000 to 2,000 to 1,500. The multiyear estimate alone cannot tell you anything about changes in any particular year in the period, only what the average value is over the full time period.
Multiyear estimates describe characteristics for a geographic area based on the geographic boundaries of that area as of January 1 of the final year of the time period. So, the 2005 - 2007 ACS 3-year estimates use the geographic boundaries as of January 1, 2007.
Using Multiyear Estimates
What types of geographic areas are published for the 1-year and 3-year ACS?
Four things you should know about how ACS multiyear estimates are produced
Choosing between ACS 1-year and 3-year estimates