This is calculated by using a 2 years 1040 personal tax returns; schedule "B" history normally. Usually any kind of income that fluctuates is most consistent with a 24 month average. If your credit scores are good, the automated underwriting system might take 1 year...that is if you are qualifying for a mortgage loan. It depends upon what the findings are and documentation requirements are...but as stated usually schedule "B" average for 24 months.
If you have less income for the prior year...the underwriter could use only the lower income average. Example: if you had $100,000 for 2008 but had only $60,000 for 2009. It is probable that the underwriter would average the $60000. They look for consistency and ongoing status...what is current most likely to continue.
I hope this helps.