Pricing SPYs on close

["Paul A." <paulha@xxxxxxxxxxxxx>]

Hi Folks:

What's the deal with backtesting closing prices of SPYs?  How are the
historical closing prices calculated?  Is it just the price of the last trade,
regardless of size or of whether it's at bid or ask?  Or is there some
standard
algorithm used for calculating the closing prices of SPYs and/or other
stocks? 
(E.g., the average of the last 5 trades of the day?)

Thanks for any info.

     -Paul