In general,the binomial probability P(X ≤ x)is approximated by the area under the normal curve to the left of x + .5.

This is known as the continuity correction for approximating a binomial distribution with a normal distribution. The area under the normal curve to the left of x + 0.5 is a good approximation for the probability of getting x or fewer successes in a binomial experiment with n trials and probability of success p, assuming certain conditions are met.