Women may face a much wider wage gap than commonly cited data indicates, earning just half of what men earn over 15 years. That's according to a new study by economists who analyzed the incomes of men and women who worked for at least one year between 2001 and 2015.

The standard annual wage gap measured by the Census Bureau shows that women make 80 cents for every dollar earned by a man. But this statistic leaves many women workers out of the picture, Heidi Hartmann, the president of the Institute for Women's Policy Research and an author of the study, tells CNBC Make It.

IWPR's methodology incorporates workforce attachment — those more "strongly attached" to the workforce are defined as working at least 12 out of 15 years full-time and year-round — and looks closely at both the consistency and long-term growth of earnings to capture a more nuanced picture of women's careers.

Census data, on the other hand, compares the earnings of men and women who work full-time in one given year. But women are less likely to work full-time on a consistent basis throughout their careers, and take more time out of the labor force to raise children, care for family or spend more time on education, Hartmann says.

The IWPR research attempts to more fully compare women's and men's earnings by taking a career-long view. The study found stark earning gaps in three periods of analysis: Between 1968 and 1982, women's earnings were 19 percent of men's. Between 1983 and 1997, they rose to 38 percent. Between 2001 and 2015, they rose, but not significantly: over that 15-year period, women made 49 percent of what men made.

The study also found that penalties for taking time off from work are especially high for women. For women who took one year off from work, annual earnings were 39 percent lower than those of women who worked all year between 2001 and 2015. For comparison, women who took one year off from work in the 15 years beginning in 1968 saw a 12 percent cut in earnings.

Men are also penalized for time out of the workforce, but women's losses are almost always greater than men's, the study found. Hartmann says she was surprised by the results of her own study: "I would have expected a bigger change in progress in this last 15 year period."

Despite considerable progress over the last 50 years, 43 percent of today's women workers had at least one year with no earnings, nearly twice the rate of men. Strengthening women's labor force attachment, Hartmann says, is critical to narrowing that gap.

Policies like paid family and medical leave and affordable child care can increase women's labor force participation and encourage men to share more of the unpaid time spent on family care, the study emphasizes.

"Women get paid less in the same jobs as men, which is typical, garden variety discrimination. Add to that public knowledge that sexual harassment is endemic," says Hartmann. "The fact is, we need stronger equal employment opportunity enforcement and more family support."

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