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Theta
Theta is a measure of the time decay of an option, the dollar amount that an option will lose each day due to the passage of time. For at-the-money options, theta increases as an option approaches the expiration date. For in- and out-of-the-money options, theta decreases as an option approaches expiration.
Theta is one of the most important concepts for a beginning option trader to understand, because it explains the effect of time on the premium of the options that have been purchased or sold. The further out in time you go, the smaller the time decay will be for an option. If you want to own an option, it is advantageous to purchase longer-term contracts. If you want a strategy that profits from time decay, then you will want to short the shorter-term options, so that the loss in value due to time happens quickly.
Notes and references
- http://www.investopedia.com/articles/optioninvestor/04/121604.asp
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