CHAPEL HILL, N.C. (MarketWatch) — The bull market on Wall Street is entering its seventh year of life.

That’s true, right? The problem is that few are bothering to ask that question, as seemingly everyone gets caught up in Wall Street’s celebration of the sixth anniversary of the end of the terrible 2007-2009 bear market.

But contrarians like to challenge notions that everyone “knows” to be true, since, as Humphrey Neill, the father of contrary analysis, famously once said: “When everyone thinks alike, everyone is likely to be wrong.”

And it turns out that a compelling case can be made that this bull market actually is only 3.4 years old.

Determining the bull market’s real age is of more than just academic interest. Skeptics are using the bull market’s age in arguing that a bear market must be imminent. Yet that argument loses its power if the bull market is only a little over three years old.

The compelling case that the bull market is younger than most think is made by Ned Davis Research, the Venice, Florida-based quantitative-research firm. According to its calendar of the past century’s major trends, the current bull market began Oct 3, 2011.

That’s because the company counts the market’s April-to-October 2011 decline as a bear market. And it’s hardly a stretch to agree with Ned Davis: Between its intra-day high April 29, 2011, and its intra-day low the subsequent October, the S&P 500 SPX, -1.11% fell 21.6%, the widely accepted definition of a bear market as a decline of at least 20%.

The reason that anyone quibbles with this judgment is that the Dow Jones Industrial Average’s DJIA, -0.87% drop in 2011 did not satisfy the 20% bear-market threshold. And even the S&P 500 did not satisfy it if the decrease is measured used closing values rather than intra-day readings.

But an objective interpretation of the history of bull and bear markets depends on the application of precise criteria about when bull and bear markets occur. You can’t, after the fact, pick and choose which criteria you want to apply.

For example, you should resist the temptation to devise an alternate definition of a bear market that relies on the DJIA alone and ignores the S&P 500, or focuses only on closing values and ignores intra-day readings. In the process of “eliminating” the 2011 bear market, you run the risk of ignoring other declines that you would otherwise want to consider as major bear markets.

So go ahead and let the rest of Wall Street celebrate this week. It can be our secret that this aging hen is actually a spring chicken!

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