Gold prices have been on a tear in 2016, making for some happy investors. However, some gold investors are happier than others. The price of gold has jumped 27.0 percent since the beginning of the year. In that same time, the SPDR Gold Trust (ETF) (NYSE: GLD) is up 24.9 percent. The Market Vectors Gold Miners ETF (NYSE: GDX), on the other hand, has skyrocketed 102.9 percent this year.

What is the difference between investing in the GLD and investing in the GDX? Gold miner stocks tend to be much more volatile than gold itself. This phenomenon makes sense because a 30 percent decrease in the price of gold, for example, would likely completely eradicate the margins of most gold miners and put their solvency at risk. However, a 30 percent increase in the price of gold would likely allow a number of gold miners to double or triple margins and income.

Related Link: The Crowded Gold Trade Gets Thinned Out

CXO Advisory crunched the numbers on GLD and GDX last year, comparing their relative performances from May 2006 to September 2015.

Not All That Glitters...

While the GLD’s weekly movements are almost perfectly correlated with the price of gold, CXO found that the GDX’s weekly price movements are only 64 percent correlated with gold.

In the long term, the GDX has demonstrated a slightly negative weekly alpha relative to the GLD, meaning it tends to underperform.

Finally, the GDX has generated a weekly beta of 1.62 relative to the GLD, indicating that every 1 percent change in the GLD suggests a 1.62 percent change in the GDX.

When gold prices fell 30.8 percent from 2012 to 2016, the GLD dropped 34.9 percent, but the GDX plummeted 74.5 percent.

In other words, when gold prices are going up, the GDX is the place to be. However, when they are flat or going down, the GLD is the safer bet. Fortunately for GDX investors, 2016 has been a very good year for gold.