So what's a dissertation like? The Big Day Part I
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The format of the day varies by department and/or school, but ours goes like this: 50 minute oral presentation given to the public (which usually means just people in your department, since nobody else cares. About 20 people showed up to mine on a summer Friday afternoon, which isn't bad), summarizing why you did the work, the results, and the interpretations. A short question/answer session, then you get carted off by your committee to a small room where they bend you over and take turns giving your butt paper cuts with pages of your dissertation. And that's what they do if they liked it. Kidding!
Following that, they all sign off on the dissertation if they find it acceptable. Some committee members may request that you do minor revisions, and leave it up to your advisor to oversee said revisions. Major alterations may require that the committee not sign off until the changes have been passed around to everyone again.
Two points: If the committee, the graduate school, and your advisor have all done their jobs correctly, and you show up with a pulse, you are going to pass. It is a foregone conclusion. There are ample opportunities to fix any problems that may occur before the big day, and everyone knows it. Secondly, if your dissertation is done right, it is not a confrontation. It is a roundtable discussion between experts, of whom you are the most knowledgeable (even moreso than your mentor, since you have done all the benchwork). The committee's responsibility is therefore to learn as much as they can from you by assessing the breadth and depth of your knowledge in your area of expertise, and your ability to apply it to hypothetical situations and future directions. Of course this is not painless, but it should be fair.