Because a comparison connects two or more things for the reader, you should name both things and state the comparison fully. In addition, the items compared should be of the same kind. For example, compare a person with another person, not with an activity or a situation. This sentence compares two stores:
(See also E7-c.)
Reword a comparison to specify comparable items of the same kind.
The original version of this sentence says that climbing is more challenging than companions (illogically comparing an activity to people). The edited sentence says that climbing is more challenging for Maria than it is for her companions (logically comparing one person to other people).
Reword a comparison to identify clearly and completely all items being compared.
In some comparative sentences, Standard English requires the coordinated use of as.