# What's the connection between implied vol curve of SPX and SPY?

I think there should be an obvious connection of the two implied vol curves from the SPX and SPY markets since the underlying of SPX is SP500, while the underlying of SPY is a ETF which tracks sp500 index

A very simple idea would be : with the same expiration date (while one is AM and the other one is PM), for the strike at the same log moneyness, i.e. $\log(\frac{K}{F})$, I assume the vol should be the same.

Anyone has any suggestions?

Motivation: SPY is a much tighter market, if I know the vol curve of SPY, and I can somehow convert the vol of SPY to the vol of SPX, then I would claim I get a more accurate vol curve for SPX.

• You seriously could have taken a bit more time writing the question with decent formatting and grammar. – SRKX Feb 10 '12 at 15:43
• They will be about the same but beware of the differing borrow rates. – Brian B Feb 10 '12 at 15:59
• being new here. I'll rewrite the question. Thanks for your advice. – DeepRed Feb 10 '12 at 16:03
• Hi Brian, thanks for your comment. You mean I should use different interest rates here? What I am doing now is: I get the interest rate from Eurodollar future, and use it for both market. But I use dividend yield for SPX, while make assumptions on the discrete cash dividend for SPY. – DeepRed Feb 10 '12 at 16:11
• The differences will be driven by technical details in the differences between the two markets, such as interest rates and dividend yields. Not a very interesting quant question, more of an annoying practical trading problem, kinda like the headache that is adjusting for day count conventions in fixed income. – Tal Fishman Feb 10 '12 at 16:41