I make over $2,500 a month from focus groups. Here's how I decide which ones are worth my time and how to find the highest-paying ones.

Filipe

• Filipe, a 30-year-old entrepreneur, makes an average of $2,845 a month participating in online focus groups.
• He started his own business teaching others how to make money from focus groups, after quitting his retail job for more flexibility.
• Focus groups are consumer studies by big brands who pay well for opinions on their products or services.

This as-told-to essay is based on a conversation with Filipe, a 30-year-old who goes by only his first name online and who runs his own business...