Mumbai: The Securities and Exchange Board of India (Sebi) on Monday proposed changes to the trading framework for exchange-traded funds (ETFs), introducing dynamic price bands, a new methodology for determining base prices and a pre-open call auction mechanism for commodity ETFs to improve price discovery and align trading limits with movements in underlying assets.Sebi said the existing framework resulted in a one-day lag in ETF base prices, while the fixed price bands often failed to reflect the price range of the underlying assets.It said, considering the operational challenges in usage of T-1 day closing NAV (net asset value) of the ETFs as base price, to start with, the base price for determination of price bands of ETFs would be T-1 day closing price, i.e. last 30 minutes of volume weighted average price (VWAP) of the ETF.