Putcall parity is a principle that defines the relationship between the price of European put options and European call options of the same stock, strike price, and expiration date. The formula can identify arbitrage opportunities where the simultaneous buying and selling of securities and options result in no-risk profit. I am writing this article in response to a number on inquiries why I have rarely addressed this topic in the past. The subject is a bit complicated and doesn’t directly impact our option-selling trades but, on the other hand, we can never receive too much education so here we go.

 

Put-Call Parity formula

Stock $ at expiration + put value at expiration = call value at expiration + face value of a bond that will pay for exercise of the call option at expiration

Think of each side of the equation as an individual portfolio and, if put-call parity is respected, both portfolios will be of equal value at contract expiration. If not, an arbitrage opportunity will exist.

 

Assumptions in this formula

Apply only to European style options that can be exercised only at contract expiration

  • Transaction costs not included
  • Dividends not included
  • Adequate liquidity available
  • Same stock
  • Same expiration

 

Put-Call parity with no arbitrage opportunity (data for calculations in far right column)

Put-Call Parity: No Arbitrage Opportunity

Each side of the equation or portfolio expires with equal value

 

Put-Call parity with arbitrage opportunity

Put-Call Parity: Arbitrage Opportunity

We used an interest rate of 3% just to complete the formula. The reason we use $20.00/1.03 for the bond value at the start of the trade is because the face value of $20.00 is not realized until contract expiration so the 3% interest rate must be accounted for. Note that in the brown cells the arbitrage opportunity was identified but in the yellow cells the inflows in both portfolios was the same whether share price moved up or down. The market immediately eliminates these arbitrage opportunities which apply only to institutional investors.

 

How do institutional traders make money when arbitrage opportunities are identified?

The more expensive portfolio is sold and the cheaper portfolio is purchased. In this scenario, a profit of $3.5825 per share will be a risk-free return. They manage their trades via shorting stocks and writing puts but this is not relevant to retail investors and is beyond the scope of this article.

 

Discussion

Arbitrage opportunities are not applicable to retail investors because we don’t have the tools to identify them fast enough when they occur. Since this was a topic of interest to many in our BCI community, I decided to publish this as one of the 52 articles I will author this year.

 

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