Doug Keenan has written a critique of the IPCC's handling of statistics in AR5. Suffice it to say he is not impressed.

Temperatures on Earth ’ s surface — i.e. where people live — are widely believed to provide evidence for global warming. Demonstrating that those temperatures actually provide evidence, though, requires doing statistical analysis. All such statistical analyses of the temperatures that have be en done so far are fatally flawed. Astoundingly , those flaws are effectively acknowledged in the IPCC Fifth Assessment Report (AR5) . The flaws imply that there is no demonstrated observational evidence that global temperatures have significantly increased (i.e. increased more than would be expected from natural climatic variation alone). Despite this, one of the main conclusions of AR5 is that global temperatures have in creased very significantly. That conclusion is based on analysis that AR5 itself acknowledges is fatally flawed. The correct conclusion is that there is no demonstrated observational evidence for global warming.

You can read it here.