Where do you find participants for user tests and interviews for persona building? Is it OK to recruit them via an existing web site for example? How much do you pay them?
There are companies who specialise in recruitment for usability testing, they will be able to source the correct user groups for your needs. I would generally only use this route if you need to target specialisms, for instance medical staff or lawyers. I would provide examples, but it would depend upon your locale.
In my experience, having worked with enterprise level companies and government agencies, these types of client generally have a pool of people who they use for marketing and focus group purposes. In these instances, it is possible to work with your client to source people that match your target audience or particular personas.
Alternatively, there are plenty of remote usability testing websites available on the market today. You won’t get as strong an insight as sitting and observing users in their environment or in a test lab, but can be a cost-effective and easier route to gain some insights.
Remote usability testing companies
It is also possible to recruit via an existing website. If you are going to do this, what I have found is a really nice scheme is once a transaction is made on the site, a message is provided that asks the customer if they would be interested in helping to improve the website. If the user opts to do so, you want to try and capture information about their age, sex, location, job title and computing level. Whatever information you need to capture to meet your needs in selecting the correct people for user testing. When you contact the people you do want to use for testing, it is a nice idea to make these people user advocates, asking them to share their experience of testing to the rest of the websites community. It is good practice to offer monetary rewards to testers.
On how much do you pay them, this boils down to the type of people you are recruiting. You will need to pay medical staff and lawyers their going hourly rate, whereas if your target market is less specialised, as little as $20 per person may be sufficient. Going back to my examples of enterprise level organisations I have worked with where the client has helped source testers, the recruits were just happy to be assisting in helping to test a system that would ultimately make their lives easier in their day-to-day.
Lastly, you can run guerrilla usability testing using a tool such as Silverback¹. The way I have conducted this is usually within an Agile methodology where we will be doing lots of testing iteratively over the lifecycle of a project. In these instances we are trying to drip-feed data in to our process over time, and a little bit of testing often is what we are looking to achieve. We literally pop in to coffee shops and ask people if for the price of buying them another coffee they wouldn’t mind completing a 15–30 minute usability test on a prototype. You need to find the right type of coffee shop, generally an arty type one with WIFI (and ask the proprietors permission first).
¹ Silverback is a Mac application created by Clearleft, a user experience design consultancy based in Brighton, UK. It is designed to be very easy to use. It makes use of the built in iSight on a Mac to record the users facial expressions and also records the screen. Chapters and points of interest can be marked using an Apple Remote.
Norman Nielsen has a free report with tips on how to recruit test participants. I guess that could be of inspiration too?
Standard market research recruiters will also do recruiting for you.
Another factor to think about when setting up the level of incentives is the importance of ensuring 'no shows'.
If you've had to travel to a different city / country and had to pay for a testing lab to do a quota of people - you really don't want people not turning up...
@DigiKev's answer is excellent, and I'll only add the following from experience in a small shop developing general applications (your situation may differ!): universities, gift cards, and Craigslist. Sometimes in combination!
If you are near a college or university, depending on its size and demographics you have a ready-made diverse population (and if it's not diverse, at least you have a population) that would be happy to spend an hour or two with you and your applications in exchange for a $10 or $20 gift card for food/coffee/iTunes/etc. Also, if you are in or near an area served by Craigslist, an ad placed there will also yield a pool of candidates typically in no time.
We got the idea for the latter method years ago, when that's what (at least) Google and eBay did to recruit testers (myself included! good times.) in the SF Bay Area.