Course 501: Constructing a Portfolio
How Many Stocks Diversify Unsystematic Risk?
In this course
1 Introduction
2 The Fat-Pitch Approach
3 What Do the Academics Say?
4 How Many Stocks Diversify Unsystematic Risk?
5 Non-Market Risk and a Concentrated Portfolio
6 Portfolio Weighting
7 Portfolio Turnover
8 Circle of Competence and Sector Concentration
9 Adding Mutual Funds to a Stock Portfolio
10 The Bottom Line

Once you own a certain number of stocks, you have eliminated all the unsystematic risk. When you have reached this point, there is no need to own any more stocks to diversify your risk of concentration, that is, the unique risks associated with any one stock. So how many stocks do you need to own to reach that point?

Let's hear from the experts. In their book Investment Analysis and Portfolio Management, Frank Reilly and Keith Brown reported that in one set of studies for randomly selected stocks, "…about 90% of the maximum benefit of diversification was derived from portfolios of 12 to 18 stocks." In other words, if you own about 12 to 18 stocks, you have obtained more than 90% of the benefits of diversification, assuming you own an equally weighted portfolio.

Essentially, the theory says that if you are properly diversified, on average, you will get the same return in the market as if you had bought a passive market index. So if you want to obtain a higher return than the markets, you increase your chances by being less diversified. At the same time, you also increase your risk.

It is also important to note that if you own more than 18 stocks, you will have achieved almost full diversification, but now you will just have to keep track of more stocks in your portfolio for not much marginal benefit.

While much of academia has focused on the risk of not being diversified enough, we believe that there's a practical risk to being too diversified. When you own too many companies, it becomes nearly impossible to know your companies really well. Instead of having a competitive insight, you begin to run the risk of missing things. You may miss something important in the 10-K, skip on investigating the firm's second competitor, and so on. When you lose your focus and move outside your circle of competence, you lose your competitive advantage as an investor. Instead of playing with weak opponents for big stakes, you begin to become the weak opponent.

Next: Non-Market Risk and a Concentrated Portfolio >>

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