Is the six-year-old bull stock market finally headed for a correction this month? Some historical data signal yes, but that data may not matter, says investment strategist Ryan Detrick.

"Going back 20 years, there isn’t a worse month" for stocks than August, he writes on his blog. In those last 20 Augusts, the S&P 500 index has averaged a decline of 0.89 percent. And in the months it has slid, the average decline was 4.67 percent.

"When August is lower, it can get ugly," Detrick says. Going back to 1980, no month has a lower average return when it is negative, even dreaded October.

But that doesn't make Detrick a bear: "2015 isn’t playing out like the average year," he says. The market broke normal patterns with its declines in January and March and rallies in February and May.

"Wouldn’t it be something if this trend continued and August sported a surprise rally?" Detrick says. "I continue to think a market that goes up for six years and then sideways for eight months is healthy."

The S&P 500 stood at 2,101 Tuesday morning, less than 2 percent below its record high.

Meanwhile, MarketWatch columnist Mark Hulbert says the double-digit-percentage returns enjoyed by the S&P 500 index in five of the last six years are unlikely to continue.

And why is that? "The stock market must jump over a very high hurdle if it’s to live up to historical averages in coming years," he writes. "Sky-high corporate profit margins will have to expand even further."

Without an increase in profit margins, "future earnings can grow no faster than sales, and sales growth is tied to an economy that is unable to produce growth in excess of 2 percent annually," he argues.

Assuming a dividend yield of 2 percent, Hulbert calculates an annualized real return of 3.5 percent for the stock market over the next 10 years.

Rising profit margins could push that number higher. But the three factors credited for boosting margins in recent years are unlikely to continue, Hulbert says. Those are "falling interest rates, lower corporate tax rates and a declining share of GDP going to worker compensation."

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