How are weights for a single year sample modified to weights for multi-year files?

I would like to know how the weights for single year files (such as 2011) are modified to create multi-year files (such as 2009-2011).

thank you.

The multiyear weights are generated by the Census Bureau in largely the same way the single year weights are generated. The differences are outlined in section 11.7 of this Census Document, along with a more lengthy description of how single year weights are generated.

I hope this helps.