The 4% Rule is one of the most well known ideas in personal finance. But is it really a rule? And does it apply to you?

Let’s start at the beginning. The father of the 4% Rule is a financial planner named William Bengen. Back in the early-1990s, he became frustrated with the prevailing rules of thumb for retirement planning. He found them too informal and set out to develop a more rigorous approach. The question he sought to answer: What percentage of one’s portfolio could a retiree safely withdraw each year? In Bengen’s definition, “safely” meant that a retiree would not outlive his or her funds over the course of a 30-year retirement.

The answer Bengen reached: 4%. Specifically, a retiree could build a reliable plan around a portfolio withdrawal of 4% in the first year of retirement and subsequent annual increases in line with inflation. He arrived at this conclusion after testing hundreds of hypothetical historical portfolios—evaluating different starting points and different withdrawal rates.

Since Bengen’s research first appeared in 1994, it has gained in popularity—but it’s also spurred a lot of debate. So how should you think about it?

The first thing to understand about the 4% Rule is that Bengen never intended it as a rule per se. In his paper, he is clear that 4% is just a recommendation—and only under certain circumstances. In a recent interview, in fact, Bengen, who is now retired, noted that the figure he used with his own clients was generally 4.5%. And today, with inflation so low, he believes that 5% makes more sense.

Meanwhile, some take the opposite view. Because of today’s very low interest rates, there’s now a camp that believes 4% is far too generous and that 3% or 3.5% is a better number.

Bengen recognizes the irony in how his 4% figure has taken on a life of its own. His intention was to develop a more rigorous approach that improved upon the old rules of thumb. And yet, over time, his work, which also included an entire book on the topic, has itself been over-simplified and reduced to a rule of thumb. That’s why I think it’s worth taking a closer look at this research. As you think about your own retirement plan, and whether 4% would make sense for you, below are six factors to consider:

Income: For many people, income varies throughout retirement. You might, for example, retire at 65 but defer Social Security until 70. If that were the case, your portfolio withdrawals might be well north of 4% in those first five years. But that wouldn’t be a problem if you expected them to moderate later on. In other words, your withdrawal rate need not be under 4% every year.

Expenses: For simplicity, Bengen’s research assumes that a retiree’s spending will increase linearly each year, in line with inflation. In reality, though, most people’s expenses vary throughout retirement. Retirees are generally more active—and thus spend more—during the earlier years of retirement. For that reason, it wouldn’t be unreasonable if your withdrawal rate were higher in those early years.

Assets: It’s a morbid topic, but if you have a reasonable expectation of an inheritance later in life, that might also allow you more latitude on your withdrawal rate early on.

Age: Bengen’s litmus test for a “safe” withdrawal rate was one that would allow a portfolio to last at least thirty years. But everyone’s retirement timeframe is different. If you are planning to retire very early—say, in your 50s—you might want to plan on a more conservative withdrawal rate. For instance, in Bengen’s study, a 3% withdrawal rate would have resulted in sustainable withdrawals for at least 50 years in every scenario. On the other hand, if you choose to work until your late-60s or early-70s, then you could afford a higher withdrawal rate.

Asset allocation: The 4% rule of thumb that grew out of Bengen’s paper was based on a portfolio of 50% stocks and 50% bonds. But he also spent quite a bit of time looking at the impact of alternative asset allocations, and he warned investors that 4% wouldn’t necessarily work for any portfolio mix.

Market valuation: In 2008, financial planner Michael Kitces extended Bengen’s work, adding another dimension: market valuation. His finding, as you might expect: If the market is at a relatively high point on the day you retire, a more conservative withdrawal rate would be warranted. Bengen endorses this finding and has built on Kitces’s analysis in more recent work. In today’s bull market, that’s a key point to keep in mind.

In short, the 4% Rule is hardly a rule. Like most things in personal finance, the answer that makes sense for someone else will rarely be the answer that makes sense for you. Bengen himself urges investors not to worship at the altar of 4%. In his words, “it’s not a law of nature.”