Which of the following describes a first quartile in a data set?

The first quartile of a data set, often denoted as Q1, is defined as the value below which 25% of the data fall. To find the first quartile, one typically looks for the median of the lower half of the sorted data set. This effectively captures the point where one-quarter of the data lies below it.

By calculating Q1, you're isolating the lower segment of the data, which provides an essential measure of spread and helps understand how the data is distributed. It is particularly useful in the context of box plots and other statistical analyses.

In contrast, the other options do not fulfill the definition of the first quartile: the lowest value represents the minimum and doesn't provide information about the distribution of the lower half; the highest value denotes the maximum and does not relate to the quartiles; and the median of the upper half pertains to the third quartile (Q3), which is not relevant when defining the first quartile.