Correction to: Nature Methods, published online 29 September 2014.

In the version of this article initially published, there were errors in an n value, two subscripts and a figure citation. In the sentence in paragraph 11 beginning “If variance is kept at 1.43 (c = 5, n = 5),” n should be 3, not 5. In the sentence in paragraph 12 beginning “We want to minimize Var(Xi...) = σB2/b + σC2/(bc) + σε2/(bcn) within the cost constraint K = bCB + bcCD + bcnCD,” the subscript D in the second term should be a subscript C and the one in the third should be a subscript N. In the next-to-last paragraph, the citation to Fig. 2b should be to Fig. 3b. The authors thank Ghadi Al Hajj for bringing the matter to their attention. The errors have not been corrected in the original article.