NYSE Composite Index - 1978

The following graph plots the year-to-date performance of the index for 1978 compared to the probability calculated by a random walk model.

Initial Value: 555.12 Hist. Volatility: 0.51% Annual Drift: 7% Final Value: 566.96 Percentile: 33%
Click on the image for the high resolution version.

The high to date and low to date prices of the stock plotted against a random walk model for 1978 (what's this?).