Intuitively, the center value of this interval is the weighted average of formula_41 and formula_42, with formula_41 receiving greater weight as the sample size increases. Formally, the center value corresponds to using a pseudocount of , the number of standard deviations of the confidence interval: add this number to both the count of successes and of failures to yield the estimate of the ratio. For the common two standard deviations in each direction interval (approximately 95% coverage, which itself is approximately 1.96 standard deviations), this yields the estimate formula_44, which is known as the "plus four rule".