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"Frontmatter". In: Analysis of Financial Time Series

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NONLINEAR MODELS 151prob0.0 0.2 0.4 0.6 0.8 1.0ooooo o o o o o o o o oo oo oo oo o o o1998.5 1999.0 1999.5 2000.0monthFigure 4.6. One-step ahead probability forecasts for a positive monthly return for IBM stockusing an 8-4-1 feed-forward neural network. The forecasting period is from January 1998 toDecember 1999.forecasts and the actual directions in the second subsample with the latter denotedby “o.” A horizontal line <strong>of</strong> 0.5 is added to the plot. If we take a rigid approach byletting ˆd t = 1 if the probability forecast is greater than or equal to 0.5 and ˆd t = 0otherwise, then the neural network has a successful rate <strong>of</strong> 0.58. The success rate <strong>of</strong>the network varies substantially from one estimation to another, and the network uses49 parameters. To gain more insight, we did a simulation study <strong>of</strong> running the 8-4-1feed-forward network 500 times and computed the number <strong>of</strong> errors in predictingthe upward and downward movement using the same method as before. The meanand median <strong>of</strong> errors over the 500 runs are 11.28 and 11, respectively, whereas themaximum and minimum numbers <strong>of</strong> errors are 18 and 4. For comparison, we alsodid a simulation with 500 runs using a random walk with a drift—that is,{ 1 if ˆrt = 1.19 + ɛˆd t =t ≥ 00 otherwise,where 1.19 is the average monthly log return for IBM stock from January 1926 toDecember 1997 and {ɛ t } is a sequence <strong>of</strong> iid N(0, 1) random variables. The mean andmedian <strong>of</strong> the number <strong>of</strong> forecast errors become 10.53 and 11, whereas the maximumand minimum numbers <strong>of</strong> errors are 17 and 5, respectively. Figure 4.7 shows thehistograms <strong>of</strong> the number <strong>of</strong> forecast errors for the two simulations. The results show

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