Chapter 10 Analyzing Change over Time

Previous chatpers do not consider time in the measurement of the variables. Roughly all the data is collected at the same time (cross-sectional). Assuming that we need to measure the pereability of rock, the researchers only measure the permeability of each rock sample once, and the passage of time and its impact is not considered. However, the permeability of a rock sample may change over time. In this case, we need to measure the permeability of the rock sample at different times.

In this chapter, we examine two configuration of data: repeated measures and time series.

• In the repeated measures, the same subjects are measured at different times. For example, we may measure the height of a group of students at the beginning of the school year and at the end of the school year.
• In the time series, the same variable is measured at different times. For example, we may measure the temperature of the air at different times of the day.

The data collected over time is not independent. With repeated-measures and time-series data, we have pairs or larger sets of observations that are connected.