By 'professional' study, I mean hiring an outside company to run the test (typically $10k+). By 'guerilla' I mean hallway testing, or 'Rocket Surgery made easy' style testing.
What is the benefit of the paid-for study?
By 'professional' study, I mean hiring an outside company to run the test (typically $10k+). By 'guerilla' I mean hallway testing, or 'Rocket Surgery made easy' style testing.
What is the benefit of the paid-for study?
Preliminary remark: The list of benefits might be a bit different depending on your type of product and company size.
Benefits you can get from hiring an agency:
The question was about benefits you can get from professional agency testing. Given a good agency I think professional testing can be really beneficial, especially if the company has't investing much time in testing so far. However, I think that guerilla can also be very beneficial, sometimes even way more than agency testing. But this would be off topic as the question is about the benefits of agency testing and not comparing the two.
Access to more expertise. I'd say it was almost impossible to quantify this though.
An expert may just pick up one extra problem - but it could be the one 'killer problem' which is most important to deal with.
Trust and Credibility. You know when you pay a credible source, assuming that the consultants credentials are valid, that the results are trustworthy and reliable. You can use them inside your organization as a tool to convince management and sponsors. You will have higher chance of getting usability improvements implemented.
A guerilla test is not reliable, but it can still be accurate. The problem is that you don't know if the guerilla test is accurate or not. I would be reluctant to use a guerilla test as a tool to convince anyone. It would be shredded to pieces before I could say my first "But...".
As in many other things in life: What you pay is what you get (WYPIWYG)