We do not calculate the PCR off one expiry only, we calculate it off the next three expiries. The reason for this is:

  1. Sometimes there is significant OI in the next expiries, which cannot be neglected. A typical example is, there have been cases, even last year, there was action in Feb series of NIFTY in December. People were betting on the budget IV.
  2. In the last week of expiry, OI build up starts for the next month. That has to be accounted for.