About one in three employees at Google, Facebook and Apple is a woman. That's an imbalance that tech sector executives Sheryl Sandberg and Tim Cook say they want to change. Yet even if their companies set a target of just over half their new recruits being women, a Breakingviews calculator shows that closing the gender gap will take up to 15 years.

Silicon Valley's persistent maleness has a few culprits. One is the education system tech firms depend on to hire engineering and related jobs. Less than one-fifth of computer and information science majors are female, according to statistics from the National Center for Women & Information Technology.

Companies entrench the problem by failing to promote women or retain them. Research from McKinsey and LeanIn.Org found that at 200 companies surveyed, women made up 36 percent of entry level positions in the technology sector but just 27 percent of middle-management positions. The figures were worse for positions at vice-president level or above.

There's one decisive way to make a difference: for companies to set clear gender targets when they hire. Say Google, Facebook and Apple committed to 51 percent of new staff being women - pretty close to the overall makeup of the labor market. Based on the rate their workforces expanded last year, and assuming one in five existing workers quit and are replaced annually, it would take Apple 15 years to reach parity. Google would do it in 14, and Facebook in a faster-but-still-slow seven years.

Small steps make a big difference. Set a truly bold goal of six in 10 new hires being women, keeping all else constant, and all three companies would achieve parity within six years.

The idea of setting targets might sound draconian and overly simple. Tech companies will say they hire people because of their talent not their gender. There are all sorts of factors that could impede getting more women in the workplace. A low rate of staff turnover, while good in itself, would slow the pace of change. But Silicon Valley needs to start somewhere, or noticeable change will take decades. Setting out a clear, numeric goal for hiring, and sticking to it, might be the most innovative thing these giant disruptive companies can do.

The author is Jennifer Saba, a Reuters Breakingviews columnist. The article was first published on Reuters Breakingviews. bizopinion@globaltimes.com.cn