An important question facing all commuters is "how long will it take me to get to work?" I know from years of experience that if I leave home at 6:30 AM, on average I will arrive at the parking garage at 7:45 AM. However, on any given day, I rarely arrive at exactly 7:45. Why? Sometimes I have bad luck and hit a lot of red lights in town. Sometimes I have good luck and there is no traffic backed up on the highways anywhere on my route. On a typical day, a random factor that happens to slow me down (a particular red light, a particular spot on the highways where traffic is slow, bad weather) is balanced out by some other factor that allows me to go faster (a particular green light, no people pulled over for speeding or accidents, good weather) and I get to work about 7:45 AM, plus or minus about 5 minutes. About once a week, I have worse than average luck (all the lights are red, there are several places along my route where there are accidents) and I am more than 10 minutes later than 7:45. Every couple months I have really bad luck with many factors working against me I'm more than a half hour late. Once every year or two, there is a hazardous waste truck that's turned over and the roads are shut down, or there is an ice storm and cars are driving 20 miles per hour, or some other improbable sequence of events happens that makes me more than an hour late.
Thus, if you asked me how long it takes me to commute to work, the best answer would be to express it with reference to probabilities rather than giving a single value. My mean travel time to work is 75 minutes, but about 40% of the time, my actual commuting time differs by 75 minutes by more than 5 minutes. About 15% of the time, my actual commuting time differs by more than 10 minutes. About 5% of the time, my actual commuting time differs from the mean by more than a half hour. Less than 1% of the time my actual commuting time is longer than an hour. So I can describe the variation in my commuting time by relating it to the probability of having bad/good luck and having many random factors working against/for me at once.
Now, let's say that I have a very important meeting at 7:30 AM. When should I leave home to make sure that I'm not late to the meeting? There is no particular time that I can leave and be 100% certain that I will get to the meeting on time. No matter what time I choose to leave, there will be some probability that I will be late. So I will choose a leaving time that reduces the probability of being late to an acceptable level, e.g. if I leave at 6:00 AM (allowing an extra half hour) there is less than a 5 % chance I will be late. If I want to be more sure than that about being on time, I will have to leave earlier.
In a similar manner, by now you each now about how long it takes you to get from your morning starting spot (whether in Commons or elsewhere on/off Campus) to your first class. You also know various factors that can hold you up: the elevator stopping at every floor, it is raining, it is colder than you thought (you turn around, go and grab a jacket), crossing a given street and using the crosswalk (safety first!), your roommate taking extra time in the shower, whether the professor actually starts something meaningful to you or not for the start of class, etc,.. Thus, you know the time you need to leave to get the day started well on time...and also when you need to try to hurry! Most days, one of those variables happen but not such that affects your results (i.e., getting to class on time). And for some of you (not all of you), sometime everything possible that can happen, will happen to cause you to be late.
