Hedging
Hedging is a method for reducing risk where a combination of assets are selected to offset the movements of each other. For instance when investing in a stock it is possible to buy an option to sell that stock at a defined price at some point in the future. The combined portfolio of stock and option is now much less likely to move below a given value. As in diversification there is a cost, this time in buying the option for which there is a premium.
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