The famous Vanguard finding that investing all at once beats spreading it out is real, but it answers a question most regular savers never face — and using it as an excuse to sit in cash has been costly.

• Vanguard's research showed investing a lump sum beats spreading it out about two-thirds of the time.
• That study assumes you already have a pile of money to invest, which most people don't.
• Money arriving with each paycheck can't be invested before you earn it, so the real choice is investing steadily versus holding cash.
• Holding cash lost badly against investing steadily — a saver waiting on the sidelines with Microsoft made far less than one who kept buying.

Outlook: For anyone investing out of a paycheck, steady buying stays the practical default, and waiting for a better entry point keeps costing more than it saves.