Business schools and consultants have long preached that writing a formal business plan greatly improves a start-up's odds of success. But a growing number of academics are questioning whether that's really the case.

Budding entrepreneurs can spend months, sometimes years, polishing elaborate 50- to 100-page business plans that include financial projections, market research, and intricate details on day-to-day planning and organization. But skeptics say there's little concrete evidence that extensive planning is highly correlated to success.

A more practical approach, they say, for entrepreneurs who aren't seeking external start-up financing from venture capitalists or angel investors, is to write a "back-of-the-envelope" plan with basic financial projections, such as cash flow, and fine-tune the business model after launching the business.

By making a business plan, you will just reduce the risk by researching many factors.

'Just Do It'

"What we really don't want to do is literally spend a year or more essentially writing a business plan without knowing we have actual customers," says William Bygrave, an entrepreneurship professor at Babson College in Wellesley, Mass., who says he generally advocates "just do it." Entrepreneurs must be nimble, and will be more apt to stick with a flawed concept they spent months drafting, he adds.

The doubts fly in the face of a huge, lucrative business-planning industry, chock full of authors writing step-by-step guides and consultants selling business-planning services. A recent search for "business plan" on Amazon.com turned up more than 19,000 book titles. Business schools also have reason to push planning: It's much easier to teach than intuition. Many colleges and universities have ramped up their entrepreneurship programs in recent years and introduced business-planning courses. Many top business schools including the University of Pennsylvania Wharton School and Harvard now host student business-plan competitions for cash prizes.

If you don't have enough experience, just search for it...