The new methodology was introduced today and allows investors to "more readily compare risk-adjusted performance" between all Morningstar-rated funds, it said.
Jackie Beard, director of closed-end fund research for Morningstar UK, said: "We wanted to make the change ahead of the industry’s shift to a fee-based advice model when a greater interest in investment trusts is expected to emerge."
Ratings for closed-end funds are now calculated using the fund’s NAV, which Morningstar said it believes to be a superior measure of its risk-adjusted performance.
Prior to today, the ratings for closed-end funds were calculated using the fund’s market closing price.
In addition, Morningstar will now calculate quantitative ratings for all London-listed closed-end funds using its own category system rather than the AIC’s sectors.
It said this would help provide analysis of a fund against its "true peers" and would be particularly important for funds with a specialist remit, which do not have a vast peer group to measure against.
Beard added: "The changes to our closed-end fund rating methodology are intended to assist advisers in their whole-of-market proposition.
"Risk-adjusted performance comparisons – on a vehicle-agnostic basis – are now possible, which means investors and advisers can now more readily compare the Morningstar ratings of investment trusts with those of similar open-end and ETFs."
The new methodology calculates the rating on the fourth business day of each month and does not adjust for transaction costs.
