powerpuff wrote:I often get confused in Comparison questions. Will be glad if some expert can help me out whether the first sentences in the examples below are correct or not

The economy of Country X is stronger than Country Y.
The economy of Country X is stronger than that of Country Y

During 1800-1900 AD, John Milton wrote more books than any other author
During 1800-19900 AD, John Milton wrote more books than did any other author.

I know that the second constructions of the sentences above are correct, but are the first constructions wrong because in general the first constructions of the two examples above are what we usually read in the newspapers etc. ?

the first case is a very clear cut error: you're comparing the economy of country X to country Y, instead of to the economy of country Y. make sure you compare two things that are comparable (apples to apples, not apples to porches):
either compare economies, or compare countries. Note that merely adding 's at the end of country Y will also solve the problem.

The second case is an error, but not so clear cut. In certain configurations, the first sentence will be ok :
"John Milton is taller than any other author" does not really require a verb, as it compares John Milton to other authors (on the basis of height).

The GMAT Sc displays a preference for fuller comparisons (including both the second object of comparison, and the conjugated verb "did" to maintain parallelism), but in some questions we see omissions, or ellipsis, in the right answer as well. (check out OG 12th ed Q93). From what we can see, in those cases the "fuller" answers are constructed in such a way that eliminates them for illogical comparison - comparing two things that are not comparable, leaving the elipsis answer as the least worst answer choice".

In other words, between the two choices you've just presented, the second sentence is preferable, but the first sentence may find itself the correct answer if compared to the following version instead:
During 1800-19900 AD, John Milton wrote more books than did any other author's.

Thanks Geva. That cleared one of my big doubts in Comparisons!