1) You may be trying to do too many questions. OG13 plus the two supplements = about 1,500 questions. If you've gone through them all but your score is still at 650, then you're not learning all of the things that you need to learn when you study. If that's the case, then plowing through more new questions isn't going to change much.
On average, I spend about 3 to 5 times as long analyzing a problem as it took to do the problem in the first place. If I spent 2 minutes to do it, I'm looking at 5 to 10 minutes to analyze - sometimes longer. And I'm pretty efficient; most students take longer. :)
So I'm going to guess that there's probably more that you could be doing there. When you finish studying a problem, do you know not just how to do that problem but how to recognize when a
different problem tests similar things? Can you recognize when parts of problems test the same things and can be solved using the same processes, even when the problems don't look all that much the same on the surface?
Have you thought about ways in which they could twist the problem or change the details so that you'd have to do something sort of similar but ultimately different? How are you going to spot the twist and know how to handle it?
If you aren't doing all of that, then part of your time should be spent going back and figuring those things out for problems you've already done. When you're ready for new problems, you can use the GMATPrep question pack to make problem sets. Just make sure that you really are extracting full value from the problems you do!
Next, what are your timing issues?
(If your immediate reaction is that your timing is fine, check again. I run across about one person a year who truly doesn't have timing issues. Everyone else does, even though many think they don't...)
2. GMATPrep are the best, but you're right to save that one for about 1-2 weeks before the real thing. Yes, use that one to decide whether you're ready.
After that, our tests are the next closest indicator, as far as I can tell. (Our standard deviation from last MGMAT CAT to real CAT is about 50 points, compared to 30 points for the real test. No other company has published this data, leading me to believe that their numbers aren't better than ours. :)
If you think that ours are unrealistically difficult, then I'm going to guess that you struggle more with quant than verbal, that you do have timing issues on quant at a minimum, and that you struggle more with computation than with other pieces of the math.
If all of that is the case, then part of what you need to learn is to cut yourself off when the going gets tough. The real test may not give you as much annoying computation, but you will have stress that you don't have on practice tests (because you know it will count now) and you will feel more pressure to hang on longer to problems you really shouldn't be doing. We can't fool you into thinking that a practice test really counts, but we can still construct things to put you under the same kind of stress - and see whether you can still make good decisions or whether the process starts to break down. So don't dismiss our tests - after GMATPrep, they really are the best.
I haven't done tests from other companies (I manage the content of our own tests, so I literally can't look at other company tests for competition / IP reasons), but I've heard fellow teachers say good things about 800Score tests. So those are the next best ones.
3. I just got to your point 3. Yes, so this is part of why you're through with the questions (point 1) and looking for more. You actually do need to buckle down and try to learn this stuff rather than going for more new questions.
I've written an article (a long time ago now) about that voting districts problem; see here:
http://www.beatthegmat.com/mba/2009/11/ ... s-problemsAnd see what I did at the end? You can use this problem (and the other one in the same article) to learn how to tackle max / min problems in general.
Next, you made a mistake along the way. Figure out what the mistake was and what you could do differently in future to minimize the chances of repeating that mistake.
eg: before you start solving, figure out the general steps you need to take and jot down short notes for each step, one note per line on your scrap paper. Then follow your steps.
In general, follow the process discussed in this article:
http://www.manhattangmat.com/blog/index ... ms-part-1/Have a plan before you start solving; if you can't come up with the plan, it's time to bail. Start looking for others' solutions to and discussions of problems to help give you ideas of the kind of things you could be doing or thinking about. Also, come back here and tell me what your take-aways for max/min problems in general based on that problem.
When was your last practice test? It's possible that your level is still around 650, but it's also possible that it has dropped in the 4-5 months that you weren't studying much. If your level has dropped, you need to figure out what specifically has gotten rusty so that you can get it back up to speed. A practice test would tell you those things.
In general, anyone looking to lift a score significantly (and I consider 50+ points significant) should be planning to study 10 to 20* hours a week. I can't tell you that you need to do this for 6 weeks because I don't know how much time you'll need to get to your goal. It may be 4 weeks or 6 or 10 or longer - it can vary greatly from person to person. You'll just have to see how it goes as you do it.
*The more you're studying in an effective way, the more you can cut this to 10-12 hours a week. Don't just toss hours at the problem. You're asking the right kinds of questions here about how to study - just keep that up and keep pushing towards how to study effectively.
Plan as though it's an exercise program. You're going to study daily, with one day off a week. (I like to make Friday my day off, my reward for studying all week long. If I slip up, though, I have to stay in on Friday and study.) Your study can be spread across a day (half an hour at breakfast or lunch if you're more of a morning person, flash cards at 10-minute intervals throughout the day, an hour when you get home from work). On days you don't work, try to sit down for a solid 1-2 hours once or twice a day (with a substantial break in between if you do 2 study sessions in a day). Turn off your phone, close your browsers, and focus on studying!