 Chapter three of our book is about measures of central tendency, or really how to be concise and find a single number that can stand in for an entire distribution of numbers. It's an exercise in simplifying, of boiling things down to their essence. Now the reason we learn this, the reason we learn about these three measures which are the mode, the median, and the mean is because every single other thing we do in this class is based off of them you start by looking at what's the single most representative and then going off from there. So if we want to calculate variability or correlations or anything else we first have to know where the center of the distribution is. Also because if you think of Occam's razor and the simplest thing the center the single most representative number is usually the one you want to go with. Now in terms of what for, what does this matter to you? Well if you're selling products you might want to look at what's the most common one that people buy, that's the mode, or if you're a physical therapist you might be interested in knowing how many sessions do people need before 50% of them recover from their injury. That would be the median and for the mean you might want to look at what's the average income of people who become social workers in Utah County. That average, the standard one, is the mean. All of these can be useful. Now the important thing is in a nice symmetrical data set, I'm drawing a little bell curve here with my hands, those three numbers will all be the same but if you have a skewed distribution like most of the ones are low but a few go way high up and if you look at stuff like number of physical therapy appointments or the cost of houses really anything to do with money or hospitalization it's going to be skewed and in those cases you'll find those numbers can be very different. So understanding the difference between them and knowing how to choose one or the other in terms of what is most representative for your purposes is the major goal of this chapter.