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Report of Referee A -- LF15808/Poh 
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Poh et al. report in their manuscript on the experimental violation of 
the CHSH inequality using a parametric down conversion experiment. 
They violate the classical bound of 2 and come close to Tsirelson's 
bound of \sqrt(2)*2: S_experiment-\sqrt(2)*2=0.008+-0.00082. They 
exceed a novel bound, which has been calculated by Grinbaum, by 2.72 
standard deviations. Their experiment is performed without addressing 
the known experimental loopholes in Bell inequality violation 
(detection loophole, signaling loophole etc.). 

The authors only consider statistical errors originating from 
Poissonian statistics for calculating the experimental uncertainty. A 
major claim of the paper is to measure precisely the violation of the 
CHSH inequality and, thus, to approach Tsirelson's bound with very 
small uncertainty and exceeding Grinbaum's bound. Therefore, in order 
to hold this statement, also systematic errors have to be considered. 
Systematic errors will usually both increase the uncertainty and also 
give bias to the determined value. A publication in Phys. Rev. Lett. 
claiming to rule out an "effective description" of quantum mechanics 
should conform to a sensible error analysis of the data. This is 
usually the case in any precision experiment, see e.g. the CODATA 
paper (P. Mohr et al., Rev. Mod. Phys. 84, 1527 (2012)) or the recent 
EDM paper (The ACME Collaboration, Science 343, 269 (2014)). 
Therefore, I cannot recommend publication in Physical Review Letters. 
As such the paper is more suited for Physical Review A. 

Another comment: The novelty of the manuscript of Poh et al. hinges on 
the violation of Grinbaum's bound. Without this, the experiment by Poh 
et al. is just another experiment violating the CHSH inequality. 
Therefore, the authors should put effort in explaining the origin of 
Grinbaums bound more in detail. Currently, the authors write that the 
violation of Grinbaums bound rules out that "quantum mechanics is only 
an effective description of a more fundamental theory". What are the 
assumptions of Grinbaum? How does he derive his bound? What are the 
implications? The work by Grinbaum [Ref. 2] should be summarized in a 
comprehensible way in the work of Poh et al. to be understandable to 
the general physics community. Only a proper explanation of Grinbaum's 
bound allows the reader to judge the validity of the statement that 
"quantum mechanics is only an effective description of a more 
fundamental theory".