Understanding Backtesting

What backtesting is, how it works and why it's valuable for retirement planning.

Last updated: Aug 2026

Quick answer

Retirement backtesting applies your plan to real historical returns to show how it would have performed in past market conditions. It helps identify worst-case periods, sequence risk and historical success rates. Backtesting is most useful when combined with Monte Carlo simulations to cover both historical and hypothetical futures.

What is Backtesting?

Backtesting tests a retirement plan against historical data to see how it would have performed in the past. Instead of assuming a constant return (like 7%), backtesting uses the actual year-by-year returns from history.

Example: A 30-year retirement starting in 1929 would use the actual returns from 1929-1958, including the Great Depression crash and recovery. A 30-year retirement starting in 1966 would use 1966-1995 returns, including the stagflation era.

With 95+ years of data (1928-2025), we can test dozens of different 30-year periods, showing how your plan would have fared through every historical scenario.

How Backtesting Works

  1. 1

    Pick a starting year

    For example, 1929 (right before the Great Depression)

  2. 2

    Simulate each year using real returns

    Year 1 uses 1929's actual return (−8.3%), Year 2 uses 1930's (−25.1%), and so on

  3. 3

    Apply withdrawals according to strategy

    Subtract the annual withdrawal (e.g., 4% of initial balance, adjusted for inflation)

  4. 4

    Check if portfolio survived

    Did the portfolio last the full 30 years, or did it run out of money?

  5. 5

    Repeat for all starting years

    Run the same simulation starting in 1928, 1929, 1930... through 1995 (for 30-year periods ending in 2025)

  6. ✓

    Calculate success rate

    If 115 out of 124 simulations succeeded, that's a 93% historical success rate

Benefits of Backtesting

Tests Real Volatility

Captures the actual sequence and magnitude of historical crashes and recoveries

Identifies Worst Cases

Shows exactly which historical periods would have been most challenging

Compares Strategies

See how different withdrawal methods perform in the same historical conditions

Builds Confidence

A plan that survived the Great Depression and 2008 crisis provides reassurance

Limitations of Backtesting

Past ≠ Future

Historical performance does not guarantee future results. The future could be worse than any historical period.

Survivorship Bias

The U.S. was one of the best-performing markets of the 20th century. Other countries experienced total losses.

Overfitting Risk

Optimizing too closely to past data may not work in the future. Simple strategies often outperform complex ones.

Limited Scenarios

We only have ~124 different 30-year periods. There are scenarios we haven't seen (e.g., prolonged deflation + low returns).

Notable Historical Scenarios

1929-1958: Great Depression Start

The worst possible start. Stocks dropped 80%+ in the first three years. Yet a 4% withdrawal rate still survived the full 30 years thanks to the post-war recovery.

1966-1995: Stagflation Era

Often considered the "true worst case" for retirees. High inflation (up to 13%) combined with poor stock returns meant real purchasing power eroded badly. The 4% rule barely survived.

1982-2011: Golden Era

One of the best periods to retire. Low inflation, strong stock returns. Even aggressive withdrawal rates succeeded. This period is often criticized for making people overconfident.

2000-2009: Lost Decade

Two major crashes (dot-com and 2008) in one decade. Stocks had negative real returns over the full period. Still ongoing in terms of long-term outcomes. We'll know more in 2030.

Backtesting vs Monte Carlo

Backtesting

  • Uses actual historical sequences
  • Captures real correlation patterns
  • Limited to ~124 scenarios (30-year)
  • Shows what did happen

Monte Carlo

  • Uses randomized returns
  • Can generate 1,000+ scenarios
  • May miss real-world patterns
  • Shows what could happen

Recommendation: Use both! Backtesting shows how your plan would have performed in real historical conditions. Monte Carlo stress-tests it against scenarios that haven't happened yet.

Next step: run your own backtest

Get your baseline with the free retirement number calculator, then backtest your plan across historical periods and compare success rates.