Compute the Wilson score interval
Mathematica, 48 bytes (UTF-8 encoding)

({-1,1}√((s=1.4^4)##/+##+s^2/4)+#+s/2)/(s+##)&amp;


Unnamed function taking two arguments in the order n_s, n_f and returning an ordered pair of real numbers. The three-byte symbol √, representing the square-root function, is U-221A.

Uses the fact that preceding ## by a number results in the product of the two arguments, while +## results in their sum. Also uses the fact that products and sums automatically thread over lists, so that {-1,1}√(...) implements the ± in the formula. Defining the constant s = z^2 instead of z itself also saved a couple of bytes. (Mostly I'm just proud of saving a byte by noticing that 1.4^4 is exactly 1.96^2!)