56. Typographical and spelling errors can be either “nonword errors” or “word errors.” A nonword error is not a real word, as when “the” is typed as “teh.” A word error is a real word, but not the right word, as when “lose” is typed as “loose.” When undergraduates write a 250-word essay (without checking spelling), the number of nonword errors has the following probability model:
| Errors | 0 | 1 | 2 | 3 | 4 |
| Probability | 0.1 | 0.2 | 0.3 | 0.3 | 0.1 |
396
The number of word errors has the following model:
| Errors | 0 | 1 | 2 | 3 |
| Probability | 0.4 | 0.3 | 0.2 | 0.1 |