Auditor Market Share of the S&P 500

We got a question this morning on Twitter about auditor market share for the S&P 500. While we’ve done plenty of market share analyses before, we hadn’t looked at that particular group of companies.

Followers of this blog – and of the audit market generally – won’t be surprised to hear that the Big Four audit the vast majority of this market index, which covers 500 large companies by market capitalization listed on the NYSE or NASDAQ exchanges.1

As one can see in the chart below, the Big Four audit over 99% of the S&P 500, with PwC in pole position. Grant Thornton and BDO, the two firms that together with the Big Four compose the Global Six, are the only other firms that audit S&P 500 companies.

For comparison, the Big Four audit about 44.2% of all US-listed public companies.

In the next chart, we provide a histogram of auditor tenure for the S&P 500. This graph shows the number of companies with auditor tenure in the ranges indicated. For example, 64 S&P 500 companies have auditor tenure between 0 and 10 years, and 172 have auditor tenure between 10 and 20.


On average, US-listed companies have auditor tenure between 16 and 23 years, with quite a few having a continuous relationship with their auditor for 100 years or more.

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1. The S&P 500 herein reflects the most recent constitution of the index, updated monthly. Auditor market share is based on data available on as of 2/28/2017.