Nearly all of the major stock market indices are "capitalization-weighted" (one notable exception is the Dow Jones Industrials Average, which is "price-weighted").

That means swings in the value of the largest constituent members have a proportionately greater influence on the price of the relevant benchmark than price changes in the shares of smaller companies.

In many cases, exchanges and financial services firms have also created "equal-weighted" versions of some of the more popular bellwethers (many of which also serve as the basis for certain exchange-traded funds, or ETFs).

With this type of structure, the equivalent percentage move in the price of a share of any company in the index, regardless of the firm's size, will have an identical impact on the value of the benchmark.

Because of the way each index is calculated, one can get a feel for whether or not a market trend has been broad-based, and thus "healthy," by examining the ratio of the price-weighted index to its capitalization-weighted equivalent.

In the eyes of many analysts, market moves that are narrowly-based often lack long-term staying power.

Based on an analysis of the equal-weighted version of the S&P 500 Index relative to its capitalization-weighted equivalent over the past five years, there is evidence of a transition that began a year ago that might call into question current expectations for much more upside in the year ahead.

Up until last summer, the rally in the U.S. market was fairly broad-based, as the ratio kept pace with moves in the benchmark index. Since then, however, the measure has flattened out and, recently, has begun to roll over, signaling that breadth is narrowing despite the S&P 500's push to new all-time highs.

For some, that divergence lends further credence to the notion that the current leg of the bull run is somewhat suspect, and seemingly at odds with what is happening in credit markets and the real economy. Under the circumstances, investors should remain on their toes.

Michael Panzner is a 25-year veteran of the global stock, bond, and currency markets and the author of Financial Armageddon: Protecting Your Future from Four Impending Catastrophes and The New Laws of the Stock Market Jungle: An Insider's Guide to Successful Investing in a Changing World.