Setting Investment Return Assumptions for Retirement

What types of returns should we assume for our retirement projections?  This is a question that has come across my desk a few times over the past several weeks.  Making reasonable assumptions about what our investment performance could be in the future is a critically important part of good financial planning.  If we assume too little, it may cause us to pursue investments that aren’t suitable for our plan.  If we assume too much, it could cause us to over-spend and potentially run out of money down the road.