And we got for this random variable with this probability distribution, we got an expected value or a mean of 2.1. What we're going to do now is extend this idea to measuring spread. And so we're going to think about what is the variance of this random variable, and then we could take the square root of that to find out what is the standard deviation. The way we are going to do this has parallels with the way that we've calculated variance in the past. So the variance of our random variable X, what we're going to do is take the difference between each outcome and the mean, square that difference, and then we're going to multiply it by the probability of that outcome. So for example, for this first data point, you're going to have zero minus 2.1 squared times the probability of getting zero times 0.1. Then you're going to get plus one minus 2.1 squared times the probability that you get one, times 0.15.