How do companies like Intellius get information on people?

I have found some wrong information from companies like Intellius, etc. on people's date of birth/age, income, and locations.How do companies like Intellius get information on people?Phone books and public records.Too many people have the same or similar names.I found at least 4 people with the same first and last name as me.All that differs is middle initial.How do companies like Intellius get information on people?They have access to databases from phone companies, utilities and others. I found them to be surprisingly effective, but of course, they can have wrong information, especially when people have moved or when there are similar names.