We often collect data across different places and time points and across human characteristics. A census collects data across different states. In a longitudinal study, we collect information over different time points. Those individuals could be male or female, and their occupation could be different. All individuals under any study could be split into different groups based on these geographical, temporal, and occupational characteristics. We usually analyze data as a whole, but sometimes it is useful to perform some tasks separately among different groups.

As an example, if we collect details of the income of different individuals from six different regions, then we might be interested in seeing the income distribution among different professions (considering five different professions), across six regions. This income could vary depending on whether the person is a male or female. In this situation, we can conceptualize this problem by...