I'm reading *Thinking, Fast and Slow* by Daniel Kahneman and I came across the following text

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> Some years ago I had an unusual opportunity to examine the illusion of financial skill up close. I had been invited to speak to a group of investment advisers in a firm that provided financial advise and other services to very wealthy clients. I asked for some data to prepare my presentation and was granted a small treasure: a spreadsheet summarising the investment outcomes of some twenty-five anonymous wealth advisers, for each of eight consecutive years. Each adviser's score for each year was his (most of them were men) main determinant of his year-end bonus. It was a simple matter to rank the advisers by their performance in each year and to determine whether there were persistent differences in skill among them and whether the same advisers consistently achieved better results for their clients year after year.
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> To answer the question, I computed correlation coefficients between the rankings in each pair of years: year $1$ with year $2$, year $1$ with year $3$, and so on up through year $7$ with year $8$. That yielded $28$ correlation coefficients, one for each pair of years. I knew the theory and was prepared to find weak evidence for the persistence of skill. Still, I was surprised to find that the average of the $28$ correlations was $0.01$. In other words, zero. The consistent correlation that would indicate differences in skill were not to be found. The results resembled what you would expect from a dice-rolling contest, not a game of skill. No one in the firm seemed to be aware of the nature of the game that its stock pickers were playing. The advisers themselves felt they were competent professionals doing a serious job, and their superiors agreed.
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Kahneman continues and claims that the financial industry is largely based on the *illusion of skill*.

**Question:** Why would this example show that the financial industry is based on the illusion of skill/that the stock picking requires no skill? I understand that the correlation between the rankings in different years says something about the relative skill of the stock pickers. That is; how the skill of stock picker $A$ compares to the skill of stock picker $B$. But I don't understand why it would say anything about the skills of the stock pickers as a group.

Suppose that you have a group of golfers that are all exactly as skilled as Tiger Woods. If you would calculate the correlation coefficients of their succes over eight years, you should get zero correlation as well, but that doesn't imply that they are weak players/have no skill.