A new batch of big players in the tech industry have joined the White House's effort to lift the earnings of women by pledging equal pay for their female workers.

The companies including General Motors, Apple, Facebook, Target, and Visa have signed a pledge with the White House to conduct an annual gender pay analysis to eliminate unequal compensation based on gender bias.

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Taking the pledge, which was first introduced at the United States of Women Summit in June, entails many commitments. Besides seeking to close the national gender pay gap by conducting annual, company-wide paymant analysis, the companies also vowed to review their hiring and promotion practices.

Earlier this year, while announcing that it has no pay gap, Apple promised to dig even deeper into compensation. A press release by the company said, "We're now analyzing the salaries, bonuses, and annual stock grants of all our employees worldwide. If a gap exists, we'll address it."

The new signees were announced in a White House statement on Friday - which was the Women's Equality Day, the anniversary of the ratification of the 19th amendment, which gave women the right to vote. These 28 new signatories that bring the total number of companies that have made the pledge to 57.

Other employers joining the White House pledge include A.T. Kearney, Anheuser-Busch, Coca-Cola, CVS Health, Delta Air Lines, Dropbox, Dunkin' Brands, EY, The Hartford, Hershey, Hilton, IBM, IKEA US, Intel, LinkedIn, Microsoft, Nike, Patagonia, and Unilever.

According to figures from the US Labor Department, women working full time on average earn 79 percent of what men do. According to studies, one of the main causes for this such is the fact that women are in lower-paying fields, and those who have children often fall further down the pay scale because of their reduced working hours.