I'll post to this thread because this is very relevant for the hiring companies. Hunter and Schmidt did a meta-study of 85 years of research on hiring criteria. [1]
There are three attributes you need to select for to identify performing employees in intellectual fields.
  - General mental ability (Are they generally smart)
    Use WAIS or if there are artifacts of GMA(Complex work they've done themselves) available use them as proxies. 
    Using IQ is mostly illegal[2] in the US, so you'll have to find a test that acts as a good proxy.

  - Work sample test. NOT HAZING! As close as possible to the actual work they'd be doing. Try to make it apples-to-apples comparison across candidates. Also, try and make accomidations for candidates not knowing your company shibboleth.

  - Integrity. The first two won't matter if you hire  a sociopath.
     There are existing tests available for this, you can purchase for &lt; $50 per use.

This alone will get you  65% hit rate [1], and can be done inside of three hours. There's no need for day long (or multi-day) gladiator style gauntlets.
[1] <a href="
[2] The illegality comes from IQ tests disadvantaging certain minority groups.
Ask HN: Who is hiring? (September 2016)