I have the following EOD options data for the SPY containing IV data for each strike.
Date Symbol Exp Strike P/C ImpVol 2015-07-01, SPY, 2015-07-10, 185.5, C, 0.272986 2015-07-01, SPY, 2015-07-10, 186, C, 0.267097 2015-07-01, SPY, 2015-07-10, 186.5, C, 0.261214 2015-07-01, SPY, 2015-07-10, 187, C, 0.255573 . .
I'd like to calculate the IV for the SPY Option Chain using this data.
I believe the Option Chain IV is related to the ATM strike IV, but I'm not 100% sure how ThinkOrSwim calculates it.
Is there a formula I can use to calculate the Option Chain IV?