TWO years ago the British government introduced regulations requiring all companies with at least 250 employees to provide data on the gender pay gap among their workers. The deadline for compliance expired on April 4th, after more than 10,000 firms had submitted numbers. They showed that, on average, the hourly wage for the median female employee is 12.1% less than that for the median male. (The difference at The Economist Group is 29.5%.)

However, the data do not show whether companies are paying women less than men for doing the same work, which has been illegal in Britain since 1975. Quantifying this bias requires a more granular picture of the exact jobs and salaries of all employees at a firm. One such study last year found that the average pay gap in Britain between men and women with the same role at the same company was a mere 0.8%. Yet even these analyses may understate subtler forms of discrimination, especially within the knowledge economy. It is harder to compare the jobs of two consultants than two factory hands.

Instead, the new data illustrate what might be called a “gender seniority gap”: women get fewer of their companies’ highest-paying jobs than men do. On average, 30.8% of female employees are in the bottom quarter of their firms’ payrolls, whereas just 20.2% make it to the top quarter. Around 30% of the differences between the sizes of companies’ gender pay imbalances can be explained by the share of their female workers in the best-compensated roles—a large amount of predictive power for a single variable.

Deciding on whether to promote women is the most important factor that determines a firm’s pay gap, but far from the only one observable in the data. The next-biggest influences are a company’s industry, size and overall share of female employees. The gap in median wages is especially big in mining, construction and finance. It narrows in larger companies, perhaps because there are more middle managers. And it widens in firms that have a greater share of female employees, since the additional women often find themselves at the bottom of pile.

Even after incorporating all of these factors, the new government figures still leave more than half of the variance between companies’ pay gaps unexplained. A disparity in skills might be a problem in certain industries: for example, easyJet cannot easily promote its cabin crew (69% of whom are women) to become pilots (who are 94% male). Discrimination probably accounts for some of the “missing” variance as well. But female employees seeking proof that they are being paid less than their male colleagues for the same work may have to wait for better data. There is greater transparency in Finland, Norway and Sweden, where workers can look up what their colleagues earn, which gives them more bargaining power.

Nonetheless, the exercise has confirmed that men command a vastly disproportionate share of plum jobs in Britain. That should force companies who thought the gender pay gap was yesterday’s problem to re-evaluate their hiring practices.