T-distribution v Normal

Hi,

When calculating confidence intervals, how do I know whether to use t-distribution or normal?

Thanks

 

One Sample

For binomial (p) and poisson (λ) always use Z.

For mean (μ) use

- Z if population s.d. (σ) is known.
- t if population s.d. is not known.


Two sample

For binomial and poisson always use Z.

For two means (μ1 an μ2) use

- Z if σ1 and σ2 are known.
- t along with pooled sample variance if σ1 and σ2 are not known but can be assumed equal.

For paired data always use t.

 

That clears it up.

Thanks!