If you have an investment portfolio, it is prudent to review this regularly to ensure it remains appropriate. Part of this review would likely involve a comparison of the portfolio to something, the question then becomes what do you compare it to?
With my clients I generally start with their financial plan as the benchmark, i.e. is the portfolio generating the returns needed to help them achieve/maintain their desired lifestyle without fear they will run out of money. For me this is very important as my experience is having peace of mind around money is an objective whereas trying to obtain a specific return above a specific benchmark is not.
Focusing on retirement income, the main issue is ensuring that the purchasing power of money is maintained/increased so it is then a sensible idea to compare the portfolio to inflation (can I buy at least the same in the future with my money or hopefully more?). This is why we invest – to provide financial security for our future selves.
After this, it is sensible to compare the portfolio to something similar to see if it remains suitable compared to its peers or whether something alternative could be considered. Here the firm’s Investment Philosophy is important, in the words of David Booth from Dimensional Fund Advisors:
"The important thing about an investment philosophy is that you have to have one you can stick with."
Therefore, in summary, there are many ways to compare a portfolio but starting with the most relevant and client specific (i.e. the financial plan) I believe to be the most meaningful approach.