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 error, $$E = z _ { \alpha / 2 } \cdot \frac { \sigma } { \sqrt { n } } \text { or } E = t _ { \alpha / 2 } \cdot \frac { s } { \sqrt { n } } ?$$