Bit weird modifier usage , any help please

I would definitely agree that it's a GREAT long-term habit to read high-quality writing for an hour or two every night. More exposure to good writing will inevitably help you better at everything on the GMAT verbal section.

The only trouble is, reading something like the Economist or any number of other publications (Scientific American, Atlantic, New Yorker, New York Times, academic journals, etc.) is a long-term strategy. It's a great way to strengthen your fundamental reading skills (which will definitely help with CR & RC), and maybe make you a little bit better at distinguishing between shades of meaning in different sentences (which should also help with SC). But that's a slow process. It's worthwhile if you're going to be studying for a while, but it won't necessarily pay off quickly.

And for SC, the GMAT tests its own version of the English language. There are absolutely zero publications out there that follow the exact same rules as GMAT SC does. The English language is constantly evolving and changing, and two different editors will inevitably have different opinions about the rules that are tested on the GMAT.

So reading is definitely good! But no single publication can be taken as an authoritative proxy for GMAT SC rules, unfortunately.

In GMAT SC, which is followed by a comma as it shows that it is a non-essential modifier that adds some information to a sentence but if removed it does not make much difference.

GMAT Ninja
Is there any possibility in a GMAT sentence when 'which' is not preceded by a comma? (as in the above sentence from economist)

If you have a phrase with "which" preceded by a preposition of some sort ("in which", "to which", etc.), then there won't be a comma. Otherwise, "which" will generally be preceded by a comma.

The GMAT doesn't really test comma usage much, though. Editors and style experts have plenty of reasonable disagreements about when commas should or shouldn't be used -- and the GMAT mostly stays away from the issue. I can't think of any GMAT questions in which the comma (or lack thereof) is a deciding factor.