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Finance textbooks typically state that 8 to 20 stocks can provide adequate diversification for a portfolio. However, these recommendations usually assume a short time horizon such as one year. We examine 20‐year cumulative rates of return and ending wealth from an initial $100,000 investment allocated among 100 large U.S. stocks. Probability distributions obtained from simulations illustrate the shortfall risk faced by investors who own fewer titan 100 stocks. Five percent of the 20‐stock portfolios have ending wealth shortfalls exceeding 28%. These findings suggest that 8 to 20 stocks may be insufficient for long‐term investors.

Intraday volatility characteristics throughout the trading week are examined at the emerging Borsa Istanbul (BIST) stock exchange. Using five-minute (and 15-minute) intervals, accentuated intraday volatility patterns at the microstructure level are examined during the stock market open and close in the morning and in the afternoon sessions. Volatility is highest when markets open in the morning. The second highest is during the afternoon open. The third highest is before the market closes for the day. Volatility before the market close has increased in recent years. These characteristics are seen every trading day. There are also differences: Monday returns are lowest, Friday returns are highest, and Monday morning volatility is highest of the entire trading week. Day-of-the-week and intraday accentuated volatility smile anomalies are jointly investigated using the longest intraday sample period in the emerging country stock exchange literature. Investment companies and professionals can utilize the results for risk management and hedging by avoiding highly volatile opening and closing periods. Arbitrageurs, speculators, and risk takers should trade during these highly volatile periods. Heightened volatility is increased difficulty in price discovery, thus inefficiency. Market participants, exchanges, and public prefer efficient markets. The research presents evidence of trading days, and periods during the trading day, when the exchange becomes more efficient. This is the first research that explores day-of-the-week effect from intraday volatility perspective in an emerging market, and provides useful recommendations in designing risk management strategies at market microstructure level.