New Delhi, Sept. 17 -- Imagine two friends visit Delhi. One goes in May and returns complaining about the unbearable heat. The other visits in December, raving about the pleasant winter chill. Both experiences are accurate yet completely contradictory, and neither can prepare you for what to expect if you visit the city at random.

Investing works in much the same way. Your returns depend on when you 'visited' the stock or fund. If two people start and end at different points, their experiences will be completely different, even if they both have identical investments.

Why portfolio-based judgements fail

When you compare the returns of one fund in your portfolio against another's, you're using your own investment timeline as the benchma...