The standard error of the mean based on a sample of size n assumes that the population is infinite or at least so large that the effect of withdrawing items during the sampling process has a negligible effect. If the size of the sample, n, becomes a not insignificant fraction of the population, N, then the finite population correction $\sqrt{\frac{N-n}{N-1}}$ is used to adjust the estimate of the standard error, which becomes $\sqrt{\frac{N-n}{N-1}}\times \frac{\sigma }{\sqrt{n}}$. It is common to use this correction if the sample size is more than 5% of the population, and it reduces the standard error, so resulting in a narrower confidence interval for the population mean.