There's a sub-set of the self employed that are often called "contract" workers. These people are in a slightly different position than the rest of us: they are employees for a specific task or period of time, and then they either move on to the next contract or are unemployed. Often they work through staffing agents, though some find their own work.
I don't think of these people as being self employed, but many of them refer to themselves as such (though they probably can't when doing taxes). I also think it's the worst of both worlds: putting up with employers while working, and suffering the uncertainty of income when you are not. Still, many people do contract work, and there is an excellent online book if you are considering this.
You can't mix contract work with real self employment unless the contract gigs are very, very short: if you are tied up for weeks or months as an employee, you can't pay attention to other customers and can't do much marketing. I've seen self employed people get caught up by that; business is slow, so they take a contract job, but then they are doing nothing for their business, so they end up having to take another contract.
I'd rather stay completely independent.
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