I've been banging my head against this very issue on and off for a couple years now.
Based on my own experience and that of others gained through some less than perfect hires (and the problems created thereby) I now use this method:

First I run two interviews of increasing complexity that do their best to cover a wide range of linux related admin topics.
Questions are always phrased in a way that require a few sentences to answer rather than...
"Do you have any experience with NFS?"

During these interviews I'll obviously will eliminate based on incorrect answers but more than anything I'm looking at personality, composure, body language and how follow up questions are answered like:
"And if that didn't work, what would your next troubleshooting step be?........And what if that didn't work either?.....And...."

The point is that I'm not data-mining, I'm method-mining.
I really don't care what a person knows or what degrees or certs they have.
I want to know:
1. Can they work in a team?
2. Can they figure out new things without having their hand held through every step of the process?
3. Will they follow through?
4. Etc

For instance I interviewed several guys last year who ran hugely important systems at AT&T, IBM, CNN, and none of them made it past the first interview.
Reason being that their "trouble-shooting-path" if you will, didn't extend anywhere beyond
1. Read the big red book the vendor sent us
2. Call the vendor

Which brings me to the final interview.

This 3rd interview is done entirely online and involves connecting via vnc to the candidate's desktop.
The candidate is given login credentials to a server used for testing.
Various prefab'd problems need to be fixed and tasks need to be completed.
Most importantly, everything is open-book, open-google, open-whatever.

In this interview I hope to stump the candidate so that they have to go search around in real time.
That they may not know the answer immediately doesn't really matter to me.
I get to watch them construct a complex command, or decide what terms to throw at google, or dig through system resources to decide what is causing a particular issue.
In essence, I get to see how their thought process works as they research.

Having this remote connection is the golden ticket to really figuring out if the person on the other end knows what they're doing.
It can also be rather nerve racking for the candidate but there's some value in that also given that it's pretty important to be able to keep a cool head when conditions are less than ideal.

-s

Matt Graham wrote: 
From: alex@crackpot.org
  
Quoting Joshua Zeidner <jjzeidner@gmail.com>:
    
The Question is... how does an employer measure Linux expertise 
objectively?
      

With great difficulty.  (So some of them look for the RHCE label.)

  
earning an advanced degree does show a person has  
commitment, and should have a good general understanding of the  
subject.  But none of that is a guarantee.  I've met a lot of  
knotheads with masters degrees (in many fields).
    

Yep.  There are even a number of complete idiots with PhDs out there.
I've seen them; my father's a professor, so I met a number of ...
less-than-useful people in the department who all had PhDs.

  
You find out what people know, and what they're good at, by talking to  
them and finding out about what they've done in the past.  It's not  
objective by any means, but I think it's also less artificial than  
trying to establish arbitrary tests or benchmarks.
    

This is true.  However, it's also harder to reduce this to a nice little
number and put it into a happy chart.  I have seen researchers, HR
departments, bean counters, and ordinary folks just freeze up and
have a cow when it comes to data that has to be interpreted in a more
time-consuming fashion than "normal" data.


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