Probability of touch
タッチ確率。満期前のいずれかの時点で株価が権利行使価格や損益分岐点などのある水準に到達する確率。その水準を超えて終わる確率のおよそ2倍にあたり、早期権利割当リスクや取引の管理時期を左右する。
Probability of touch is the chance that the underlying trades through a given level — a strike, a breakeven or a target — at any single moment before expiration, not just where it settles on the last day. Because the stock has the whole life of the option to reach that level, touch probability runs far higher than the chance of finishing beyond it: as a rule of thumb it is roughly double the probability of expiring past the strike. A 16-delta short option, for instance, ends in the money about 16% of the time but gets touched closer to 30%.
In practice, premium sellers use it to judge how often a position will come under pressure. A strike with a 30% touch probability means that even though the option is likely to expire worthless, price will brush or breach it in nearly a third of cases, and that is exactly when early-assignment risk climbs and a defensive roll starts to look tempting. Knowing the number up front lets you set management rules before the trade rather than reacting in the heat of a move.
The common mistake is confusing a touch with a finish. A level being touched does not mean the trade is lost — the stock can trade through your strike and drift back by expiration — so treating every touch as a stop-out leads to over-managing and paying away your edge. Use touch probability to anticipate stress, not to define the outcome.
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