Finding percentiles in a data set

Intro

Howdy!  I'm Professor Curtis of Aspire Mountain Academy here with more statistics homework help.  Today we're going to learn how to find percentiles in a data set.  Here's our problem statement: Use the following cell phone airport data speeds (in megabytes per second) from a particular network.  Find the percentile corresponding to the data speed 4.1 megabytes per second.

So here we have our data set.  Let's click on this little icon to the right so we can put our data into StatCrunch.  StatCrunch will make this really easy to find the percentiles.  I'll show you how this works.  First, we're gonna resize this window for a better view of what we're doing.  OK, now that our window is resized, let me show you how to use StatCrunch to make finding percentiles really easy.

Sort the data

The first thing you need to do is sort the data.  Now, fortunately for us, our data set here is already sorted.  But if we didn't have it sorted, we could just come up here to Data, click on Sort and then we tell it we want to sort the data.  And then it would actually sort the data for us in a separate column which we could then use to find the percentile of interest.  Because our data is already sorted, we don't need to do that.

Find the percentile

So the value of interest that we’re asked to find the percentile for is 4.1.  So I come over here to my list that's sorted and find the first appearance of 4.1.  Scrolling down here, I see 4.1 first appears on row 30, so I want to take the number just before it — which is 29 — and then I want to divide it by the total number in the data set.  So if I scroll down further, I can see there's 50 values in the data set.  So I want to take 29 (which is the row just before the first appearance of the value of interest, 29) divided by the total (which is 50).

So back out here on my calculator, I'm going to take 29 divided by 50 — and I can multiply by 100 to convert from decimal to percent — and I get 58.  Nice work!

And that's how we do it at Aspire Mountain Academy.  Be sure to leave your comments below and let us know how good a job we did or how we can improve.  And if your stats teacher is boring or just doesn't care to help you learn stats, go to aspiremountainacademy.com where you can find out more about accessing our lecture videos or provide feedback on what you'd like to see.  Thanks for watching!  We'll see you in the next video.