Number of points on the interior of the convex hull of a random subset This question is in regards to the following problem:


  Suppose you are given a set $S$ of $n$ points in the plane. Let $R$ be a random subset of $S$ of size $r$ with all subsets of size $r$ equally likely. What is the expected number of points of $S$ that lie on the interior of the convex hull or $R$? I.e. What is $E[|S\cap \operatorname{int}(\mathcal{CH}(R))|]$?


I have been looking in Clarkson and Shor's Applications of Random Sampling in Computational Geometry, II, Mulmuley's chapter in the handbook of computational geometry, and related papers. As far as I understand it, these methods all apply to bounding the number of points of $S$ outside the convex hull, but do so by finding the expected sums of the sizes of conflict lists. For instance, the conflict list of an edge in the problem above is the number of points in $S$ that are beyond it, and the sum of the sizes of all conflict lists is $O(n)$. But because the same point may appear in $O(n)$ conflict lists itself, this doesn't seem to say anything about the number of points on the interior. 

Any help in understanding the problem, or useful references is appreciated. Unfortunately, I have a sneaking suspicion that the answer is obvious, but find myself a bit stuck. Thanks.
Computing the probability that a point is inside a random convex-hull is quite interesting, and got some attention recently (under a slightly different sampling model). The exact probability can be computed (but not too quickly). Good estimates follow by looking on the Tukey depth of the points of interest. See the following recent paper, and references therein.

http://sarielhp.org/p/14/prob_ch/