You just go out and buy the information. It's not really available to individuals.

https://www.bloomberg.com/news/articles/2017-10-02/equifax-h...

Think about all the times you've told a recruiter/hiring manager your previous salary (don't, btw). Think about the HR teams and such at your company that have access.

Shrewd people can get this data, especially if they're willing to pay for it.

There are market research firms that specialize in this area. It's useful to have such information, so it has monetary value - money presumably saved by underpaying or avoiding overpaying your workers. It's not something the average employee would afford in money and effort to research.

It's a good question how the data is obtained. Maybe through mutual data sharing agreements in exchange for access? Anyway, I've see such reports for my country, with very detailed breakdowns for job types, seniority, broad technology area, forecasts etc.

Companies use information services in order to pay appropriately. They neither want to overcompensate and drive higher wages nor do they want to under compensate and lose staff to competitors.

You can get pretty reasonable numbers, especially in the software industry, just using the freely available H1B visa salary data. All employers using these visas have to disclose the title, salary and location for an employee hired on one. In my experience for companies I'm familiar with these numbers are usually pretty accurate.

> https://h1bdata.info

There are companies out there who organize salary surveys.

Every company pays in a bit to cover the costs/overhead with putting together a market report, and also submit the job titles and salaries of their workers. Possibly more data too, but I'm not certain. E.g. time with the company or years of relevant experience would be very handy.

Check out imercer.com. Depending on how you look at it it may start to look like a a middleman for cartels.