“You can only trust the results when you interview customers who match your target profile. Even if your friends and family happen to fit the profile, there’s another big problem: They’re biased, or at the very least they know too much. In your test, you’re looking for honest reactions from real-world customers – something you can never get from someone who knows you.” 

— A quote from the book “Sprint”

Miles Cheverton's answer

Miles is working as a Service Design Consultant at the Department for Environment, Food and Rural Affairs in the UK. Here is what he had to say about it when I shared this book quote on Linkedin:

Depends on loads of things. For one, if they are in the next room and you want to run a pencil sketch past them as the first pass for obvious clangers, why not? 
If they’re, you’re entire cohort, obviously not :) everything is context isn't it?

My two cents

If you can recruit people that are not your direct friends or even get a company to help you do that, awesome! But sometimes, time and money are in short supply. In those cases, one trick that can work pretty well is to ask friends of friends. Ask a friend to recommend someone else that would fit your criteria but that you don't know. Sure these people would still have some positive bias towards you, but it's hopefully less intense. 

More important, these are people that you don't know, so you are less biased towards them.

This can help you to avoid reactions like these in your mind: "Oh, I know why he is saying this". And instead, as you don't know, the person is curious about their reactions and feedback.

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