Basically, count all the AV given, divide by all the AV received. The risk ratio is a metric you can use to estimate the riskiness of the trade.
For what it's worth, 7 of the 8 trades of 2012 have about the same risk. The Justin Blackmon trade was substantially less risky than the other 7.
The Mo Claiborne trade was in the middle of the pack in terms of risk. That, in my mind, puts a serious end to Dan Graziano's complaints about Mo. If he keeps whining about the Mo Claiborne trade, he has to whine about the Fletcher Cox trade as well. They have essentially the same risk.