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A huge part of Nasdaq risk comes from six stocks

Apple's stock has seen a rough summer, dropping from 132 to 114 in just a couple of weeks. As its volatility has been increasing, the tech giant's effect on the broader Nasdaq 100 index is also growing.

Through Tuesday, Apple's weight in the index was 12.99 percent, but it's contribution to short-term risk was 17.04 percent.

Those numbers are from Axioma, a firm that creates tools to help portfolio managers with risk modeling and investment strategies. Axioma's risk measure is a forecast for how volatile the stock will be relative to the overall index in the next two months.

The 17 percent risk number is in line where it was last week—at 18 percent, but higher than where it ended the last two years:

(Tweet this chart)

While Apple's risk proportion has been moving upward in the last two years, if you look at a broader time period, it's nowhere as big as it once was—and could be again. Look at 2010 and 2012 when Apple represented about a quarter of the entire index's volatility.

The six horsemen of the Nasdaq

Beyond just Apple, a few other names have started to add outsized weight to the index.

Consider Google (both GOOG and GOOGL), along with Amazon, Facebook, Gilead and Netflix. They now represent 34 percent of the total weight of the Nasdaq 100, but a whopping 40 percent of the risk.

Think about that: 40 percent of the risk in the index comes from just six of 100 stocks.

Those six stocks have seen their impact grow in the past few months. At the end of 2014, they only contributed 33 percent of the index's total risk profile.

So what does it mean?

When somebody is thinking about buying or selling the tech index overall, through futures or an ETF, it's important to consider what actually goes into that index. What are they actually trading? At these levels, it increasingly looks like the whole index moves based on these six stocks, with "everything else" almost like portfolio gravy. Buyer beware.