The relationship between readership and circulation is a complex one. Long-term trend data show a strong relationship between the two, but there can be changes, both sudden and gradual.
For instance, the chart below shows the changing relationship between readership and circulation for Reveal magazine as the number of readers-per-copy increases over time.
In some cases, sudden period-on-period changes are a result of sample variation, particularly for smaller titles. However, around half of the period-on-period changes in readers-per-copy are statistically significant and it is important that the NRS reflects these real changes in reader-per-copy.