If the assumptions are wrong, the conclusions aren't likely to be very good.

Related:

• The conclusions of most good operations research studies are obvious.
  Robert E....
• Any theory can be made to fit any facts by means of appropriate additional assumptions.
  Robert E....
• There comes a time when one must stop suggesting and evaluating new solutions,
  and get on with the job of analyzing and implementing...
• The explanation requiring the fewest assumptions is
  the most likely to be correct...
• Good salesmen and good repairmen will never go hungry.
  Robert E....
• The explanation requiring the fewest assumptions is the most likely to be correct.
  William of...
• Fast personal decisions are likely to be
  wrong...
• You're reasoning is excellent-
  it's only your basic assumptions that are wrong...