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Kelly ratio
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What this is
The Kelly criterion answers one question: given a win rate and a payoff ratio, what fraction of a bankroll maximises its long-run growth rate? For a trade log it is f* = W − (1 − W) / R, where W is the share of decided trades that won and R is the average winner divided by the average loser.
Two things it is not. It is not a forecast — it is arithmetic on the trades already in this log, and it assumes the next trade is drawn from the same distribution as those. And it is not a number most people bet: full Kelly is the growth-maximising fraction, which means it is also the most volatile one you can hold without losing growth, so half and quarter Kelly are shown beside it because that is what the fraction is normally scaled to in practice.
Sample size dominates everything here. A win rate measured over 20 trades moves several points if one trade flips, and Kelly amplifies that error. Treat a thin sample as a description of what happened, not a measurement of an edge.