Dissertation Stress: How Good Does the Research Have To Be?

The first chapter of my dissertation, the introduction, is written. The next three chapters, one each for the first three experiments, are drafted - data collected, results written, and only in need of refinement. So why do I feel so stressed out, and so unsure about whether I'll be ready to defend and graduate this year?

It all comes down to the final experiment. Data collection progresses slowly, for reasons not all in my control; it will certainly extend to January, and might extend to February - giving me only a month or less to write up that experiment and the general discussion. So whenever I think about the dissertation, I focus on the question of how demanding my committee will be. Will three successful experiments, and a reasonable attempt at the fourth, be enough? Or will they insist that the fourth experiment must also be of publishable quality?

Last year, one of the students declared that she was not at all worried about her defense because she already had a post-doc offer in hand, and none of the faculty were going to hold her back. I suppose that if I don't have a job offer myself, I shouldn't care if I need an extra year to pull off the final experiment, but I can't stand the thought of failing my defense. Surely three of four successful experiments should be good enough; but do I really want to graduate as just "good enough"?