Comparisons require both elements to be parallel. When you see comparison words or phrases such as “more than,” “less than,” “although,” “rather than,” etc, check to make sure the things being compared are grammatically parallel.

Incorrect:
The professor published more papers last year than were published by all his colleagues combined.

Correct: The professor published more papers last year than all his colleagues combined.

For comparisons, the grammatical forms need to be balanced rather than identical. This means that while you need to include the same parts of speech in both elements of comparison, they do not necessarily need to be in the same order. The correct sentence above has one “noun + active verb” construction and one “active verb + noun” construction (the verb “did” is implied: than (did) all his colleagues combined).

Just as you can’t compare apples to oranges, you can’t compare two things with different grammatical structures.
Sometimes, you’ll come across comparisons between multiple pronouns or a noun and a pronoun. In many cases, in order for the pronouns to be parallel, the pronouns must be identical.

Incorrect: Those who exercise in addition to maintaining a healthy diet are likely to be in better health than the people who maintain a healthy diet but don’t exercise.

Correct: Those who exercise in addition to maintaining a healthy diet are likely to be in better health than those who maintain a healthy diet but don’t exercise.

Here, people who exercise are being compared to people who don’t exercise. In the first sentence, the pronoun “those who” in the first part of the sentence is matched with the noun “the people who” in the second part of the sentence. Notice how much cleaner and easier to understand the second sentence is, where the pronoun “those” stands in for “people” in both parts of the comparison.

Use the same pronoun for both elements of the comparison. Consider the sentence below:

Those who have strong work credentials and a college degree are more likely to be hired than one who has only the degree.

This sentence compares two types of people, but uses two different pronouns: “those” and “one.” This confuses the basis of comparison. The pronouns must match:

Those who have strong work credentials and a college degree are more likely to be hired than those who have only the degree.

OR

One who has strong work credentials and a college degree is more likely to be hired than one who has only the degree.

Both sentences are grammatically correct. It does not matter which pronoun you choose to use; all that matters is that they match, and that the verbs in the sentence agree with the chosen pronoun (“those” requires plural verbs, whereas “one” requires singular verbs).

Incorrect Incorrect: Those who have strong work credentials and a college degree are more likely to be hired than one who has only the degree.

Correct Correct: Those who have strong work credentials and a college degree are more likely to be hired than those who have only the degree.

Correct Correct: One who has strong work credentials and a college degree is more likely to be hired than one who has only the degree.

Be consistent: whichever pronoun you choose, use it all the way through.