quantum mechanics - Delayed choice entanglement swapping: why are Alice & Bob's measurements useless without Victor's?

Here is the article by Ma et al.:http://arxiv.org/abs/1203.4834

I have read many explanations on this site and others that emphasize that Victor's data is needed to make Alice and Bob's usable... Folks frequently mention: "that Victor's data is needed to sort the data from Alice and Bob's trials into four subsets..."

I'm not a quantum physics native, so I do not understand why Victor's data is needed. Can somebody please explain in very simple terms:

1. why you cannot deduce correlation/entanglement from Alice and Bob's data alone?



2. whether it would make any difference to the above question if you can repeat the experiment (and collect many more data points from Alice and Bob) either through multiple parallel set ups of the same apparatus, or multiple measurements in a single apparatus, but before Victor does his thing?