Limitations of the Trinity study ...
The authors of the paper, however, did not mean for their scenarios to be applied rigidly or uncritically. The article makes this very important statement:
The word planning is emphasized because of the great uncertainties in the stock and bond markets. Mid-course corrections likely will be required, with the actual dollar amounts withdrawn adjusted downward or upward relative to the plan. The investor needs to keep in mind that selection of a withdrawal rate is not a matter of contract but rather a matter of planning.
Nisiprius requested clarification from Professor Philip L. Cooley, senior author of the Trinity study:
[3] What the "4% SWR" means is not that you can treat a portfolio as if it were a guaranteed annuity. I think all the [Trinity] authors meant is that if it is late 2008 and your stocks halve in value, you don't need to halve your spending instantly. It's OK to cross your fingers and continue spending according to the 4%-then-COLAed plan, even though it means dipping into capital, and it's OK to go on doing that for a while.
Professor Cooley's response:
You have hit the nail on the head! I've tried to explain that thought to journalists but they don't seem to get it. You've got it. Stay flexible my friend!, which is the advice we should give to retirees.