00:01
This problem says the rate of return of stock indexes is approximately normal.
00:05
Since 1945, the standard in pors 500 has had a mean return of about 12 % with a standard deviation of 16 .5%.
00:12
Take this normal distribution to be the distribution of yearly returns over a long period.
00:17
A wants us to figure out what range do the middle 95 % of all yearly returns lie.
00:21
And then for b, we're asked if the market is down for the year, if the return on the index is less than zero, what percent of years is the market down? and to start off with a, since we want the middle 95 % and we're dealing with a normal distribution, we can use the empirical rule, which tells us that the middle 95 % would fall between the mean and plus or minus two standard deviations.
00:41
So in our case, that's the mean 12%, plus or minus two times the standard deviation of 16 .5, which will give us 33.
00:50
And when we subtract, we'll get the lower boundary of the middle 95%, which gives us negative 21%.
00:57
And then when we add, we'll get the upper boundary, which comes out to 45%.
01:02
So that's where our middle 95 % would lie...