Big, scary stock market sell-offs happen all the time in bull markets

The markets have been getting rocked this week.

Stocks in the US were lower for a fourth day on Friday following Thursday's brutal sell-off. The Dow was down as much as 291 points Friday with all 30 Dow stocks lower. The S&P 500 briefly dropped below 2,000 for the first time since February (all of it sectors were in the red).

Friday's intra-day low of 1,993 on the S&P represents a 6.6% drop from its intra-year high of 2,134.

So, is it time to panic?

It's important to note that big sell-offs occur all the time, even during years when the markets are surging. For what it's worth, even the Black Monday crash of 1987 was part of a bull market.

"Despite average intra-year drops of 14.2%, annual returns [have been] positive in 27 of 35 years," JP Morgan Funds' David Kelly recently noted.

Kelly offers this chart of intra-year declines that we think every investor should take a good look at.

The bottom line is: Sell-offs happen. Sometimes they're big. But they're normal, so there's no reason to panic.

Yet.