When is it an appropriate choice in writing to use the passive voice

When the subject (person or thing) performing the action is not relevant.

Explanation:

The Passive Voice when writing or speaking is used when you wish to focus the attention on the person or thing affected by the action; instead of focusing on the person or thing who/that performs the action. I hereby give you an example.  

As you can see, the second option does not include a subject/pronoun/person, and even if it were, the most remarkable there would be the letters and not the person who performed the action, whereas in the first example, it is remarkable the fact that “they” sent the letters; not you, not me. You can as well, include a name or be more specific:

the service industry in the united states grew in the late 1800s. what was one effect of that growth?

the number of people with jobs in factories as unskilled workers decreased.

the number of people with jobs in factories as unskilled workers increased.

the middle class grew as people took jobs as clerks, lawyers, and teachers.

the middle class shrank as people left jobs as clerks, lawyers, and teachers.

a) a master's degree

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