Definition
Backtesting means replaying a betting strategy on past games to see how it would have performed using only information available before each bet. It helps separate a clear rule from a guess. The basic return formula is:
ROI = net profit / total amount risked
If a strategy risked $5,000 across historical bets and finished with $250 in profit, its ROI was 5%.
Worked Example
A bettor tests a rule: bet Team A only when their projected win probability is 55% or higher and the market price is -110.
At -110 odds, a $110 bet wins $100 if it cashes. The break-even probability is:
110 / (110 + 100) = 52.38%
The backtest finds 100 qualifying bets. Each risks $110, so total amount risked is $11,000. If 56 bets win, profit is:
56 × $100 = $5,600
44 × $110 = $4,840 lost
Net profit = $760
ROI = $760 / $11,000 = 6.91%
Why It Matters
Backtesting helps a bettor check whether a rule had measurable value after accounting for odds, stake size, and losses. It is most useful for finding weak assumptions before risking real money.