[johnlbevan2]

Another factor may be subjective vs absolute data points in the source.

"Do you work from home more than 50% of your working hours?" has a simple yes/no answer. "What is your salary, including average annual bonus, after tax?" has a single numeric answer.

"How many years have you been a developer?" sounds simple; but has interpretation issues. Do I count from my first line of code when playing as a kid, my first computer science class, my first paying job as a developer, or some other time? If this is a measure of seniority (e.g. we define senior as anyone who's been coding for 10+ years) there will be a mixed bag.

This may mean that despite trying to factor out variables they're hiding in the noise of bad data; so may still have a hand in the result. i.e. Those developers who've counted 9 years developing from their first salaried job and now earn more and work from home are in the same group as those who've just graduated but counted their time coding from their 8-year-old-self's first foray who are in their first year of the job, and thus on lower salary and less perks.