Marek Ziebinski has a full time job working for a telecommunications network. But, like many Americans, he also has a side hustle. In his free time, the 28-year-old Colorado resident rents cars to tourists through a website called Turo.

Mary O’Connor took on a side hustle after she was laid off from a job at Yahoo, but she doesn’t work full time any more. O’Connor partners with another Los Angeles-area mom so they can squeeze a few hours for their baby equipment rental business into their busy days taking care of young kids.

Ella Tyler is officially retired, but she takes occasional tutoring work to supplement her Social Security and pension income.

In many ways, Ziebinski, O’Connor and Tyler represent the 21st Century worker – entrepreneurial and mercurial, defying traditional definitions of both employee and self-employed.

While millions of people continue to work in traditional jobs, various studies chronicle the growth of a vibrant new economy made up of freelancers, independent contractors and the self-employed. Often working at full-time jobs during the day, these independent contractors supplement their incomes by working for themselves at night, on weekends, and during lunch breaks.

Experts estimate that anywhere from a few million to nearly 60 million Americans engage in what’s alternately known as the “gig” or the “freelance” economy. How can there be such a huge range? No one appears to agree on how to measure – or even define – the freelance economy. Indeed, the “Gig Economy Data Hub” – a website sponsored by researchers at Cornell University and The Aspen Institute – have a flow chart on their site that breaks down how six different studies that purport to measure this economy define what they’re measuring. No two are alike.

New studies come out nearly daily and they rarely agree. Indeed, just this week both The Hartford and the JP Morgan Chase Institute released research on the gig economy. The Hartford survey estimated that 25% of all Americans – some 57 million individuals overall – were engaged in a side business, though most were also employed elsewhere. JPMorgan’s research said only about 4.5% of Americans were freelancing through online platforms and many of those people had only a passing relationship with the freelance economy, working their side hustles just a few months each year.

What makes the freelance economy so tough to measure and define? People like Ziebinski. During the day he works full time as a manager for a technology company. However, he works from home, which allows him to use evenings, breaks and lunch hours to make a few extra bucks on the side.

Is he a self-employed or employed? Is his side business a “job” or “an investment”? Reasonable people could disagree.

He’s always liked the idea of generating passive income by renting out assets, so he bought an inexpensive car about a year ago and listed it for rent on a website called Turo. Knowing that he’d be competing with the likes of Enterprise and Hertz, his simple business strategy was to keep his rental rate a few bucks cheaper than the competition.

The car rented so well that he bought three more. He now figures that after taxes, maintenance and repairs, he clears about $250 per car per month and he’s created a corporate structure for the operation – Mark’s Cars LLC.

He doesn’t care how the world classifies him. What matters to Ziebinski is to make extra money, which allows him to save prodigiously. Ultimately, he says he’d like to retire early – maybe at age 35. And by “retire” he means work for himself in a job that’s both flexible and doesn’t require a full 40-hour week.

Tyler is equally hard to define. The 67-year old former attorney was fully retired for a while, but after her husband died, she was having a tougher time handling the repairs needed to maintain her historic late-1800’s home outside of Austin, Texas. She didn’t want to move or go back to work, but saw an advertisement looking for people who could coach students hoping to take the law school admissions test. She now works a few hours each month online, and has even tutored kids when she was away on vacation.

“All you need is a cell phone and a WiFi connection,” she says. “It’s been a lot cooler than I expected.”

O’Connor thought it would be great to give up her long daily commute in favor of being a stay-at-home mom. But when she got laid off from Yahoo, she says she discovered something new about herself: “I’m really terrible at being unemployed.”

The parent of a toddler, she had once rented baby equipment when their family was on vacation. She thought about starting her own baby rental company, but building the website, handling the marketing, securing the proper insurance coverage, collecting payment and screening potential renters seemed like too much to add to her day. She signed on with BabyQuip instead.

The site takes a portion of her income but it manages the web site and provides insurance for every rental. While BabyQuip says independent contractors earn an average of $600 per month on their platform, O’Connor says she and her business partner — a fellow stay-at-home mom –have found that their monthly earnings and expenses are too inconsistent to accurately report.

“There is no normal,” she says. “We get really busy over the summer and the holidays, but then things drop off. Every week is different.”

Her work schedule is equally unpredictable. Where some customers rent equipment months in advance, others call her from the airport, she says.

“I was at the park with my son and somebody texted me to see if I had two car seats that they could rent,” she said. “I said ‘sure’ and then we went to Target and bought them.”

Yet, where the profile of the today’s worker may not fit in a neat niche, their motivation does. Whether it’s to make up for stagnant wages, pay off debt, keep a hand in the workforce while stepping back to raise kids, or to retire early, today’s side hustlers are motivated by money. And the freelance economy seems to have a nearly endless variety of ways that they can earn it. Indeed, many participants in this freelance workforce do multiple side hustles in a single month. 

A blogger who calls himself the Financial Panther, for instance, dabbles with 14 different online job and rental platforms, including Airbnb, Rover, and Postmates.  Once a month, he reports on how much he earns in a “side hustle report,” which is also a money-making activity.

With so many side hustles to choose from, “an extra $500 to $1,000 per month is within reach for anybody,” he says.