This year has already been remarkable for the stock market because the moves seen in 2018 are unlike any in the past, according to an analysis by Bespoke Investment Group.

Bespoke said in a note Wednesday that the correlation between S&P 500 closing prices in 2018 and that of previous years is dramatically lower than usual, making this a period "so unique that it has no historical precedent."

“We have done this type of analysis many times over the years, and usually when we run it, the years that have the most similarity to the year in question have correlation coefficients of +0.85 or even greater than 0.90,” analysts at Bespoke said. “In the analysis we ran for 2018, however, there were only two years with a correlation coefficient of more than +0.50, and five of the top ten years have correlation coefficients of less than +0.40."

The most similar year to 2018 in the stock market is 1942, with a correlation coefficient of just 0.533, Bespoke found.

"It’s hard to draw too much in the way of conclusions from the results, but of the ten years where the S&P 500’s closing prices through 7/11 had the highest correlation to this year, the S&P 500’s rest of year returns were modestly above average with slightly less volatility," they said.

The is up more than 4 percent year to date but it has not been a smooth ride. At one point, the index was down more than 10 percent from an all-time high set in January.

This year has also been much more volatile than 2017. The S&P 500 has had 36 days where it rises — or falls —at least 1 percent. In all of 2017, it recorded just eight moves of such magnitude on a closing basis.

Market volatility has spiked this year as investors grapple with a number of different factors combining to make this year quite unique. A massive corporate tax cut took effect early this year that has boosted corporate earnings. U.S. inflation is also rising, however, sparking fears the Federal Reserve will have to tighten monetary policy faster than expected.

Investors are also weighing how a trade war between the U.S. and China, and other key partners, could impact corporate earnings and the broader global economy. A trade battle is something investors haven't had to deal with for quite some time. The U.S. unveiled on Tuesday a list of $200 billion in Chinese goods it is targeting for tariffs. The announcement came after the U.S. slapped tariffs on $34 billion of Chinese goods last week.