AP Statistics Practice Test – Prep, Practice Questions & Study Guide

A z-score quantifies how many standard deviations an element is from the mean of the data set. In this case, a z-score of 1.8 means that the measured acidity (pH) of rainwater is 1.8 standard deviations above the mean acidity level of the dataset.

This is significant as it indicates that the pH level is higher (i.e., more acidic) than what is typically expected (the average). A higher pH corresponds to more acidic rainwater, while a lower z-score would indicate less acidity. By understanding that the z-score reflects the relationship between the observed value and the average, we can accurately conclude that the rainwater's acidity is above average by that margin.

Thus, this reasoning reinforces why the correct choice reflects this important aspect of z-scores in understanding data distribution in statistics.