The most common mistake that occurs when comparing two things is an incomplete comparison. Below, Grammarly has identified an instance of this error:
The student has written the following:
Errors increased even more when intellectual inferiority of grades became a factor.
This comparison is incomplete because the increase in errors is compared (using more) to nothing. “Errors increased more than what?” a reader might wonder.
The student could address this error by simply completing the comparison,
Errors increased even more than in previous cases when intellectual inferiority of grades became a factor.
More information about constructing comparisons can be found in the 6th edition APA manual, section 3.09.