Aggregate basically means "all combined/together." The words that come before the word depend on what you're talking about and how you're talking about it. 1. The funds, at least in the aggregate, have underperformed to stock market. - Here, the writer's priority is to talk about how the funds haven't done well in the (don't know why it says "to") stock market. He knows that can be argued with, however, so he prefaces that by saying, "at least in the aggregate," meaning "at least the total of all the funds put together, not specific exceptional cases". He does it to clarify his point so he can find common ground with anyone who doesn't agree. I'd say he says "In THE aggregate" here because he's referring to a certain type of aggregate that has to do with these certain types of funds. However, he would be correct in using any of the other "aggregate" phrases you're wondering about. 2. Many other players have real production - which, in aggregate, can impact global supply. - Here, the author talking about how there are many countries/companies/people (which he calls "players," since the global economy is often compared to a game) who are producing a lot. Now, maybe individually, these "players" can't do much in terms of supplying the world with this product, but put together, "in aggregate," they can make a strong impact. This is serving to emphasize that power in numbers can create change. We aren't talking about specific numbers or data, so it works best to say "in aggregate," but it would be ok to use the other phrases here, too. 3. Taken in aggregate, however, the government data offer some degree of comfort. - Here the writer is probably making a point that makes something he said before more "comforting." He probably listed some individual facts that did not sound promising, but when he has "taken" all the facts available and put them "in aggregate", we see that in the big picture, everything is ok. Once again, he could use "aggregate" in those other ways, but this way serves to explain how everything he has talked about before can mean something else when looked at when it is all put together.