# using bid ask prices to imply bid ask volatilities

Let's say i have bid / ask feed of an option prices (across strikes and expiries, calls and puts), what is the accurate way of implying out vols from these bid / asks

For eg; to get the bid vol, should i be using the bid prices on calls and puts. To get the ask vol, should i be using the ask prices on calls and puts?

First of all, i will need to use put call parity (for index options) F = (c-p)/DF + K to get the forwards ( i may get slight differences depending on K) but how do i correctly apply the bid and ask option prices here? Should i use bid prices to get a bid? forward which i used to imply the vol (likewise use ask prices to get the ask forward to imply ask vol) or is this not accurate?

Assume i have access to BS model (black76) for implying vols, given price, forwards, rate, etc As you can imagine, im not a quant, just an average joe trying to improve my reporting for the traders and management.

## 1 Answer

As regards option prices, most people would use the midpoint of the spread as the fair price. Then, risk free rates would be computed by using the yield on very safe government fixed income instruments like a US Treasury Bond. You can pick two bonds with times to maturity straddling that of your option contract and you interpolate them linearly to get a risk-free rate. You could do it differently, but that's an easy one. Now, you MUST make some adjustement on the index for dividends. One way would involve using discounted realized dividends. Another way would be to impose put-call parity -- since you are only missing the dividend yield.

Once you have your cleaned up data, you can invert Black-Scholes to get implied volatility -- but you need a numerical algorithm to do it. There are many ways to get an approximate value in closed form, so you could use the approximate value to initialize a Newton-Raphson algorithm to invert the Black-Scholes formula numerically.

• ok so just work off the mid prices? – Andrew Oct 27 '20 at 4:33
• That would the most common approach to my knowledge. Loads of option pricing papers do that. – Stéphane Oct 27 '20 at 4:59
• I cannot second your argument re risk free assets. The option market‘s risk free rate is not a treasury rate but the PAI rate stipulated by the CSA/CHouse. I would stick with @Andrew’s implied estimation ansatz. If you try this for ET options you will get a splendid (!) fit. – Kermittfrog Oct 27 '20 at 6:39
• To add a bit more flesh: To me it seems as if @Andrew 's aim is to produce some reporting for traders which might require a slightly different ansatz in terms of which side of the B/A you analyse. – Kermittfrog Oct 27 '20 at 7:07
• @Kermittfrog In the literature, they do it this way or in other similar ways. But, of course, you're right: he likely has some rates already defined in house. I am not a trader, nor an expert in the field. I simply said that this was common in the few dozens of articles I read. – Stéphane Oct 28 '20 at 0:07