private double computeCumulativeProbability(int n, double p) {
        double p_inverse = 1 - (1 - p)/computeN(n);
        return studentT.inverseCumulativeProbability(p_inverse);
    }
Use StudentT distribution and estimate how many hits you need from each partition
to to get the globally top-k documents with the desired probability

@author baldersheim
container-search/src/main/java/com/yahoo/search/dispatch/TopKEstimator.java