1. The problem asks for the greatest change in the consumer price index (CPI) over the period 2015-2020 and the year it occurred. 2. The CPI change is given as percent change from the previous year for each year from 2015 to 2020. 3. From the data, the changes are: 2015 (unknown), 2016 (unknown), 2017 (unknown), 2018 (unknown), 2019 (unknown), 2020 (5.7%). 4. The greatest change is the largest percent change value among these years. 5. According to the problem statement, the greatest change was 5.7% in 2020. 6. Therefore, the answer is that the greatest change in the consumer price index was 5.7%, and it occurred in 2020.