How Do You Determine Whether to Use the Z or T

Essay

How do you determine whether to use the z or t distribution in computing the margin of $\text { error, } \mathrm { E } = \mathrm { z } _ { \alpha / 2 } \cdot \frac { \sigma } { \sqrt { \mathrm { n } } } \text { or } \mathrm { E } = \mathrm { t } _ { \alpha / 2 } \cdot \frac { \mathrm { s } } { \sqrt { \mathrm { n } } } \text { ? }$