Traders usually know their strategy's win rate and almost never know what depth of decline that win rate implies. The two are connected by ordinary probability, and the implied figure is generally larger than the one people carry around in their heads. Discovering it during the drawdown rather than before it is the common sequence, and it is the wrong way round.
Streaks Are Longer Than Intuition Says
Independent trades with a fixed chance of winning will produce runs of consecutive losses, and the length of the longest run grows with the number of trades taken. Across a few dozen trades a modest streak is likely. Across a few thousand, a streak that would feel catastrophic in the moment is not merely possible but expected. Nothing has to go wrong for it to happen.
Intuition resists this because people underestimate clustering in random sequences. A run of losses feels like a signal, a pattern, something that demands an explanation. Frequently the only explanation available is that sequences of independent events look lumpy, and a strategy taking a trade most days accumulates enough events for the lumps to become substantial.
The Win Rate Sets the Streak, the Payoff Sets the Depth
Two quantities do the work. The frequency of losses determines how long the runs get. The size of the average loss relative to the account determines what each one costs. Depth is the product of the two, and either can dominate the result.
A high win rate method carrying large individual losses produces a shallow decline that suddenly deepens, because the rare losses are individually heavy. A low win rate trend following method spends most of its life slightly underwater and depends on infrequent large wins to recover, so its typical decline is long rather than sharp. Neither shape says anything about whether the method is any good. They are different distributions that may well share the same expectancy.
Size Multiplies Everything
Position size is the only one of these you fully control, and it scales the drawdown directly. Doubling the risk per trade does not double the discomfort. It roughly doubles the depth of every decline the method produces, while doubling the gains as well.
The asymmetry lives in the recovery. A deeper hole requires a proportionally larger gain to climb out of, and the requirement grows faster than the loss that created it. That is arithmetic rather than sentiment, and it is the main reason conservative sizing survives longer than aggressive sizing at identical expectancy. It is also why a trader who increases size after a good run frequently discovers that the next perfectly ordinary drawdown, at the new size, feels nothing like the last one did at the old.
Sample Length Matters As Much As the Method
A worst case drawdown quoted from a short test is not a worst case. It is the deepest hole that happened to occur in that particular sample, and a longer sample will nearly always find a deeper one. The figure grows with observation time and never settles at a final value.
The honest way to hold it is as a floor rather than a ceiling. Whatever the deepest decline in your record, the reasonable expectation is that a worse one is still ahead. A plan that only survives the historical worst case is a plan built with no room for the future, and the future is where all the remaining trades are.
Knowing the Number Before You Need It
There is a straightforward exercise worth doing once. Take the method's win rate and its average loss, and work out what a plausible losing streak costs at your current size. Then ask whether the resulting account balance is one you could keep trading from without changing anything about the way you trade.
If the answer is no, the size is wrong today, not after the streak arrives. The point of the exercise is not precision, and any figure it produces is an estimate resting on an assumption. The point is that it converts an abstract worry into a specific balance, and a specific balance can be compared against the size you are actually trading, which is a comparison that either passes or fails rather than one you can talk yourself around.