This is the question you’re asking:
“Do I need the new edition of the textbook? Because my friend/brother/grandmother has the old edition and they gave it to me for free/for a discount/for a bottle of moonshine.”So the answer is: Yes. Yes, you need the new edition.
Put it this way: Do you want to get the best possible grade in the course? Or do you want to save money?
I know that’s a facile dichotomy. Textbooks are expensive. When a new edition comes out, there are no (much-cheaper) used copies available yet. Saving some money on a textbook is not trivial--and if you can get one free from a friend, that’s great. Some students literally do not have the money to buy new copies of the textbook for every course they’re taking. I am sensitive to the high cost of textbooks, and it’s an important factor when I’m choosing the textbook for a course.
On the other hand, textbooks (especially in science) are continually being updated with new research findings and theories. And there are exam questions drawn on newly updated material. If you don’t have the current edition of the textbook, you’ll just be guessing on those questions. They make up only a small portion of the exam (in my courses, anyway), but every mark counts.
All publishers have a “What’s New” section for every newly updated textbook they publish. This is for instructors mostly, but I encourage you to take a look at what has been changed before you make your purchase decision. Here’s what’s new in the updated textbook I’m using in my PSYCO 282: Behaviour Modification course starting in fall, 2015. It’s a fairly substantial list of changes, compared to other textbooks I use in other courses.
If you are short on funds, the good news is that the University of Alberta Libraries have started a new purchase program for reserve materials. If there are at least 100 students in a course, an instructor may ask the library to purchase the textbook for the course and make it available from their reserve room(s). (For my PSYCO 282 course, the UAlberta Libraries actually bought four copies of the new textbook, and placed them on reserve in Cameron Library.) If you can’t afford to buy a textbook, you can check the textbook out for 2 hours at a time. So you can potentially read the old edition, and check out the new one to read the updates.
Why aren’t you studying?