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The Guiding Principles and New Rules document is now in effect.
On job offers, and the declining thereof
WearingglassesOf the friendly neighborhood varietyRegistered Userregular
Unless you want to decline to the manager or person you interviewed with personally, send an email. For instance, if it was a friend, or someone you had met with a few times to discuss the job, calling and saying "I like the job but it's just not going to work for me, and I wanted to let you know personally" can be the right thing to do.
If it was one job of many and you simply didn't like it, an email to HR is all you need.
With things like this I generally think you should respond in kind. They sent you an email, so you should send them an email. You could do both, but I think that is overkill. If you only called and spoke to someone, I think there is a greater possibility that some of the people that would like to know you're not taking the job would not find out.
Posts
Just do both. Be polite and professional as burning bridges is always a bad idea.
If it was one job of many and you simply didn't like it, an email to HR is all you need.