this might be of some use to some of you....I had to search around to
find this rather practical explanation on Implied Volatility
IMPLIED VOLATILITY EXPLANATION
How to use implied volatility
to estimate how much a stock or index will move?
When the implied volatility of an option is 35%, what does that mean?
Volatility is a measure of how much a stock can move over a specific
amount of time, and is defined as the standard deviation of daily
percentage changes of the stock price. Implied volatility is simply the
volatility that makes the theoretical value of an option equal to the
market price of an option.
Volatility is usually expressed in annual terms, and represents a one
standard deviation move in the stock. Dip into any statistics book, and
it’ll explain that one standard deviation, up and down, encompasses about
two-thirds of all occurrences. So, two-thirds of the time, the stock with
an implied volatility will theoretically be between up 35% and down 35%
in one year. But how do you convert that to a more useful timeframe, when
most of the options you trade expire in the next couple of
months?
If you assume that a stock has an annualized volatility of 35%, how much
might it move in 1 day, 1 week, or 1 month? Well, open that statistics
book again and see that the standard deviation increases proportionately
to the square root of time. So, if there are 252 days in a year, you want
to multiply 35% by the square root of 1/252 to get the 1 standard
deviation for 1 day. That comes out to about 2.2%. So, in one day,
two-thirds of the time the stock will be between up and down
2.2%.
To get that number for any number of trading days from today, multiply
35% by the square root of the number of days/252. That is, for the range
5 days from now, multiply 35% by the square root of 5/252. For 20 days
from now, multiply 35% by the square root of 20/252. So, in 5 days,
theoretically two-thirds of the time the stock will between up and down
4.93%, and in 20 days, between up and down 9.86%.
You can use this data to gauge the risk of certain positions. Let’s look
at short call verticals. The stock is $80, and April options have an
average implied volatility of 40% and 15 days to expiration. Two-thirds
of the time, the stock will theoretically be between up and down 9.76% in
15 days, or between 72.89 and 87.81. So, if you’re thinking about selling
the 85/90 call vertical, there’s a pretty good chance that the short 85
calls will be in the money at expiration. If you don’t like that
scenario, but still want to be short a call vertical, you may want to
consider selling a 90/95 or a 95/100 call vertical instead. You won’t
receive as much premium as if you sold the 85/90, but there’s less risk
of the short options being in the money at expiration.
This calculation is no guarantee of what a stock will do, and is
susceptible to changes in volatility. But it is one more tool you can use
to become a more informed trader.
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