Percentiles and Quartiles (Jump to: Lecture | Video )

For the following data set:

1, 2, 3, 4, 5

What percent of the numbers are even?

Figure 1.

40% of the values are even.

Percentile
A percentile is a value below which a certain percentage of observations lie.

For the following data set:

2, 2, 3, 4, 5, 5, 5, 6, 7, 8, 8, 8, 8, 8, 9, 9, 10, 11, 11, 12

What is the percentile< ranking of "10"?

Figure 2.

Now, what value exists at the percentile ranking of 25%?

Figure 3.

Here, it is saying that the 25% percentile exists somewhere between the 5th and 6th value. There is no "5.25th" value, so I take the average of the 5th and 6th values to find what value exists at the 25th percentile. 5 and 5 average to give us an answer of 5.

Quartile
Quartiles divide data sets into quarters. There are three quartiles: The 1st Quartile is located at the 25th percentile, the 2nd Quartile is located at the 50th percentile, and the 3rd Quartile is located at the 75th percentile.

Figure 4.

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