a quant at Bridgewater told me something that flipped how i think about backtesting
"a backtest is one path through history. one. we need to see ten thousand before we trust anything"
that tool is Monte Carlo simulation. invented in 1946 by Stanislaw Ulam while working on nuclear weapons at Los Alamos
and most quants don't trust it
this video covers 6 reasons why in 90 seconds
the core problem: Monte Carlo assumes your returns are independent and normally distributed
real markets have fat tails, volatility clustering, and serial correlation
simulate without those and your risk estimates are dangerously optimistic
a standard Monte Carlo says your max drawdown is 15%
add fat tails and it's 35%
add correlation between consecutive losses and it's 48%
same strategy. three completely different risk profiles depending on which assumptions you feed the simulation
this is why quant desks don't run vanilla Monte Carlo
they run it with GARCH volatility, regime-conditional distributions, and bootstrap resampling from actual historical sequences
retail runs a backtest, sees profit, goes live
quant desks run 10,000 simulations, find the 5th percentile outcome, and ask: can i survive this path for 14 months without changing anything?
if the answer is no, they don't trade it. regardless of the median outcome
> Monte Carlo method: 1946, Los Alamos
> used at every major quant desk since the 1980s
> the 6 problems: explained in this video, free, 90 seconds
> most retail traders have never run a single simulation on their own strategy
your backtest is one story the market could have told you
there are 9,999 others. some of them end very differently
full breakdown in the video below