In real life, things depend on each other. Say you can be born smart or dumb and for the sake of simplicity, let’s assume whether you’re smart or dumb is just nature’s flip of a coin. Now whether you become a professor at Standford is non-entirely independent. I would argue becoming a professor in Standford is generally not very likely, so probability might be 0.001 but it also depends on whether you’re born smart or dumb. If you are born smart the probability might be larger, whereas if you’re born dumb, the probability might be marked more smaller. Now this just is an example, but if you can think of the most two consecutive coin flips. The first is whether you are born smart or dumb. The second is whether you get a job on a certain time. And now if we take them in these two coin flips, they are not independent anymore. So whereas in our last unit, we assumed that the coin flips were independent, that is, the outcome of the first didn’t affect the outcome of the second. From now on, we’re going to study the more interesting cases where the outcome of the first does impact the outcome of the second, and to do so you need to use more variables to express these cases.