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Worked example: Creating a box plot (even number of data points)

AP Stats: UNC‑1 (EU), UNC‑1.L (LO), UNC‑1.L.1 (EK), UNC‑1.L.2 (EK) CCSS Math: 6.SP.B.4

Video transcript

- [Voiceover] Represent the following data using a box-and-whiskers plot. Once again, exclude the median when computing the quartiles. And they gave us a bunch of data points, and it says, if it helps, you might drag the numbers around, which I will do, because that will be useful. And they say the order isn't checked, and that's because I'm doing this on Khan Academy exercises. Up here in the top right, where you can't see, there's actually a check answer. So I encourage you to use the exercises yourself, but let's just use this as an example. So the first thing, if I'm going to do a box-and-whiskers, I'm going to order these numbers. So let me order these numbers from least to greatest. So let's see. There's a one here, and we've got some twos. We've got some twos here and some threes, some threes, some four-- I have one four and fives. I have a six. I have a seven. I have a couple of eights, and I have a 10. So there you go. I have ordered these numbers from least to greatest, and now, well just like that, I can plot the whiskers, because I see the range. My lowest number is one. So my lowest number is one. My largest number is 10. So the whiskers help me visualize the range. Now let me think about the median of my data set is. So my median here is going to be, let's see. I have one, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14 numbers. Since I have an even number of numbers, the middle two numbers are going to help define my median, because there's no one middle number. I might say this number right over here, this four, but notice, there's one, two, three, four, five, six, seven above it, and there's only one, two, three, four, five, six below it. Same thing would have been true for this five. So this four and five, the middle is actually in between these two. So when you have an even number of numbers like this, you take the middle two numbers, this four and this five, and you take the mean of the two. So the mean of four and five is going to be four-and-a-half. So that's going to be the median of our entire data set, four-and-a-half, four-and-a-half. And now, I want to figure out the median of the bottom half of numbers and the top half of numbers. And here they say exclude the median. Of course I'm going to exclude the median. It's not even included in our data points right here, because our median is 4.5. So now let's take this bottom half of numbers. Let's take this bottom half of numbers right over here and find the middle. So this is the bottom seven numbers. And so the median of those is going to be the one which has three on either side, so it's going to be this two right over here. So that right over there is kind of the left boundary of our box, and then for the right boundary, we need to figure out the middle of our top half of numbers. Remember, four and five were our middle two numbers. Our median is right in between at four-and-a-half. So our top half of numbers starts at this five and goes to this 10. Seven numbers. The middle one's going to have three on both sides. The seven has three to the left, remember of the top half, and three to the right. And so the seven is, I guess you could say the right side of our box. And we're done. We've constructed our box-and-whiskers plot, which helps us visualize the entire range but also you could say the middle, roughly the middle half of our numbers.
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