In finance, investment advising, and retirement planning, the Trinity study is an informal name used to refer to an influential 1998 paper by three professors of finance at Trinity University in the US. It is one of a category of studies that attempt to determine "safe withdrawal rates" from retirement portfolios that contain stocks and thus grow (or shrink) irregularly over time.

In the original study success was primarily judged by whether portfolio lasted for the desired payout period, i.e., the investor did not run out of money during their retirement years before dying; capital preservation was not a primary goal, but the "terminal value" of portfolios was considered for those investors who may wish to leave bequests.

"The 4% Rule" refers to one of the scenarios examined by the authors. The context is one of annual withdrawals from a retirement portfolio containing a mix of stocks and bonds. The 4% refers to the portion of the portfolio withdrawn during the first year; it is assumed that the portion withdrawn in subsequent years will increase with the consumer price index (CPI) to keep pace with the cost of living. The withdrawals may exceed the income earned by the portfolio, and the total value of the portfolio may well shrink during periods when the stock market performs poorly. It is assumed that the portfolio needs to last thirty years. The withdrawal regime is deemed to have failed if the portfolio is exhausted in less than thirty years and to have succeeded if there are unspent assets at the end of the period.

The authors backtested a number of stock/bond mixes and withdrawal rates against market data compiled by Ibbotson Associates covering the period from 1925 to 1995. They examined payout periods from 15 to 30 years, and withdrawals that stayed level or increased with inflation. For level payouts, they stated that "If history is any guide for the future, then withdrawal rates of 3% and 4% are extremely unlikely to exhaust any portfolio of stocks and bonds during any of the payout periods shown in Table 1. In those cases, portfolio success seems close to being assured." For payouts increasing to keep pace with inflation, they stated that "withdrawal rates of 3% to 4% continue to produce high portfolio success rates for stock-dominated portfolios."

The authors make this qualification:

The word planning is emphasized because of the great uncertainties in the stock and bond markets. Mid-course corrections likely will be required, with the actual dollar amounts withdrawn adjusted downward or upward relative to the plan. The investor needs to keep in mind that selection of a withdrawal rate is not a matter of contract but rather a matter of planning.

Attempts have been made to study the effects of asset valuations on safe withdrawal rates and update the 4% rule and Trinity Study for longer retirements.

Other authors have made similar studies using backtested and simulated market data, and other withdrawal systems and strategies.