To solve the problem of finding the median of the given data set, let's proceed step by step:

Step 1: Arrange the data in ascending order.

The data set is: $\{7, 21, 9, 18, 15, 17, 21, 7, 14, 9, 13, 18, 19\}$

Arranged in ascending order: $\{7, 7, 9, 9, 13, 14, 15, 17, 18, 18, 19, 21, 21\}$

Step 2: Determine the median.

The median is the middle value of the data when arranged in order. For an odd number of data points, the median is the value at position: $\text{Median position} = \frac{\text{Number of data points} + 1}{2}$

Here, the number of data points is 13. Therefore: $\text{Median position} = \frac{13 + 1}{2} = 7$

The 7th value in the sorted data is: $\text{Median} = 15$

Final Answer:

The median of the data set is 15.

Would you like more explanation or have any questions?

Related Questions:

1. What is the mean (average) of the same data set?
2. How would the median change if another number were added to the data set?
3. What is the mode of this data set, and how is it identified?
4. How does the median differ from the mean in terms of resistance to outliers?
5. How do you calculate the range of this data set?

Tip:

When working with large data sets, always double-check that all data points are included when sorting!