There is a degree of subjectivity in selecting what differences to apply. The seasonally differenced data in Figure 8.3 do not show substantially different behaviour from the seasonally differenced data in Figure 8.4. In the latter case, we may have decided to stop with the seasonally differenced data, and not done an extra round of differencing. In the former case, we may have decided the data were not sufficiently stationary and taken an extra round of differencing. Some formal tests for differencing will be discussed later, but there are always some choices to be made in the modeling process, and different analysts may make different choices.