Well, imagine a world, imagine a world here that you're able to find out the true linear relationship, or maybe there is some true linear relationship between shoe size and height. You could get it if, theoretically, you could measure every human being on the planet and depending what you define as the population, it could be all living people or all people who'll ever live. This isn't practical, but let's just say that you actually could. You would have billions of data points here for the true population, and then if you were to fit a regression line to that, you could view this as the true population regression line. And so that would be y hat is equal to, and to make it clear that here, the y-intercept and the slope, this would be the true population parameters. Instead of saying a, we say alpha, and instead of saying b, we say beta times x. But it's very hard to come up exactly with what alpha and beta are, and so that's why we estimate it with a's and b's based on a sample.