I have 12 stratified samples covering different areas of a population. I know how to calculate the point estimate (PE) and confidence interval (CI) for each sample. I also know how to calculate the overall PE. However, I need to also calculate an overall confidence interval.

How do I calculate an overall CI that represents multiple samples? Do I simply calculate the CI for each separate sample and then sum them all to derive the overall lower and upper boundary?

Thank you!