most successful stock market in the 20th century. Other countries' markets displayed lower long-run returns (but still with positive equity premiums). Picking the best observation (US) from a sample leads to upwardly biased estimates of the premium. Survivorship bias of exchanges: This refers to the equity holder's fear about an economic crash such as the 1929 stock market crash eventuating, even when the probability of that event occurring is minute. The justification here is that over half of the stock exchanges that were operating in early 1900s were discontinued, and the equity risk premium calculated does not account for this. As such, the equity risk premium is "calculated for a survivor" such that if returns from these stock exchanges were included in the calculations, there may not have been such a great disparity between returns gleaned from bonds compared to stocks. However, this hypothesis cannot be easily proven and Mehra and Prescott (1985) in their studies, included the effect on the equity returns following the Great Depression. Although shares lost 80% of their value, comparisons of returns from stocks against bonds showed that even in those periods, significantly higher returns were gained from investing in stocks. Low number of data points: the period 1900–2005 provides only 105 years which is not a large enough sample size to run statistical analyses with full confidence, especially in view of the black swan effect. Windowing: returns of equities (and relative returns) vary greatly depending on which points are included. Using data starting from the top of the market in 1929 or starting from the bottom of the market in 1932 (leading to estimates of equity premium of 1% lower per year), or ending at the top in 2000 (vs. bottom in 2002) or top in 2007 (vs. bottom in 2009 or beyond) completely change the overall conclusion. However, in all windows considered, the equity premium is always greater than zero. A related criticism is that the apparent equity premium is an artifact of observing stock market bubbles in progress. David Blitz, head of Quant Research at Robeco, suggested that the size of the equity premium is not as large as widely believed. It is usually calculated, he said, on the presumption that the true risk-free asset is the one month T bill. If one recalculates, taking the five-year T-bond as the risk free asset, the equity premium is smaller and the risk-return relation