When [math]np[/math] and [math]n(1-p)[/math] are large enough, the binomial distribution can be approximated with a normal distribution.[br]Generally, the usual rule of thumb is [math]np\geq 5[/math] and [math]n(1-p)\geq 5[/math].[br][br]Note: For a binomial distribution, the mean [math]\bar{x}=np[/math] and the standard deviation [math]\sigma=\sqrt{np(1-p)}[/math][br]The probability density function for the normal distribution is [math]\frac{1}{\sigma\sqrt{2\pi}}e^{-(x-\bar{x})^2/(2\sigma^2)}[/math]