You’ve probably heard that we are currently in the longest bull market ever in history.

It began at the bottom of the markets on March 9, 2009, after a 16-month and 54 percent free fall of the S&P 500.

The question has now become, “How much longer will this bull market run?” Remember, it takes a 20 percent decline for a bull market to turn into a bear market, so what will trigger this type of sell-off and when will it occur? Or better stated, “Why is the stock market so difficult to predict?”

During this long-running bull market, some investors have sat on the sidelines holding cash, looking for the opportune time to get in.

Others have jumped in because the low interest rates were too difficult to endure while watching stocks move higher.

This begs a couple of key questions. If you’re on the sidelines, how will you know when to get in? If you’re already in, how will you know when it’s time to get out?

If the stock market was predictable, these questions could easily be answered. However, it is not.

There are several issues to consider: the fair valuation of stock prices, the events that could trigger a downturn, and the human decision-making process. Let’s take a closer look at these issues.

The actual price of a stock is determined by market activity, but how do you calculate a stock’s fair value?

Ideally, it would be based on some standardized formula. However, there are many ways to derive this figure: analyzing the company’s balance sheet, determining the net present value of a company’s future earnings, or numerous other methods.

Each method yields a slightly different result, making it difficult to know if a stock is overvalued, undervalued, or fairly valued.

Next, let’s look at the events that have occurred during the current bull market: elimination of the Federal Reserve’s quantitative easing program, the “Flash Crash” of 2010, domestic and international government debt downgrades, tumbling gold and oil prices, and the list goes on. Yet none of these events have derailed the current bull market.

So, what will be the triggering event(s) that leads to a bear market? Rising interest rates? The explosion of government debt? Geopolitical turmoil? Or maybe some “black swan” event, meaning something completely unforeseeable? Knowing which event will cause a trend reversal is nearly impossible.

And let’s not ignore the logical and emotional components of investors. We may analyze a situation using logic but when it’s time to act, we refer to our emotions.

When making investment decisions, there is an investor on the other side ready to buy what you’re selling or selling what you want to buy. You need to process the relevant data and make a good decision.

However, it’s impossible to know everything you would need to know and process it without any bias. For these and other reasons, even the most analytical individual will sometimes make subpar decisions.

Put all these issues together — accuracies or inaccuracies of stock valuations; economic, political or a number of other trigger points; and the nature of human behavior — and the outcome is a very unpredictable market.

Maybe as investors, we all need to heed the advice of one of the most successful investors in the world, Warren Buffett, who once said, “Stop trying to predict the direction of stock markets, the economy or elections.”

What we do know is that the market is unpredictable, so prepare yourself for the volatility by knowing your level of risk tolerance, realizing your time horizon for staying in your investment strategy, and following your investment plan rather than your emotions.

Wendy Bennett is a senior financial adviser in Butler.