The largest gold miner ETF is down more than 20% the past three months to fall to its lowest level against bullion prices in at least three years.

Market Vectors Gold Miners ETF (NYSEArca: GDX) has dropped 31.6% the past year to triple the loss of SPDR Gold Shares (NYSEArca: GLD).

“The gold-mining industry, which has underperformed the precious metal for each of the past six years, is pledging to report costs more accurately as part of its efforts to win back investor confidence,” Bloomberg News reports Wednesday.

“The largest gold companies are seeking to lure investors back to the $300 billion industry after a string of money-losing multibillion-dollar takeovers and over-budget projects,” Bloomberg reports, adding the firms are vowing to focus on margins and to get a grip on soaring production costs, rather than boosting output. [Gold Miner ETFs Lowest Since Sept 2009 After Fed Minutes]

A small-cap fund, Market Vectors Junior Gold Miners ETF (NYSEArca: GDXJ), is off about 42% for the trailing 12 months.

“Gold miners do not perfectly track the price of the physical commodity. Most of these companies have significant operating leverage, which can magnify the effects of fluctuating gold prices on their profitability,” writes Morningstar analyst Alex Bryan in a profile of GDX. “They also add a layer of political and operational risks that investors in the physical commodity do not face. For example, in recent years, several of the fund’s mining operators misallocated resources by running low-return mines that did not cover their cost of capital.”

GLD and other bullion-backed ETFs have made it easier for investors frustrated by underperforming miners to simply buy gold.