Panel Data
Panel data (or longitudinal data) typically refers to the general form of datasets which contain measurements of multiple “panel members”/objects over multiple timeperiods. In other words, they are datasets/tables with multiple indices.
Panel data can be presented in many ways. Using the stock market to illustrate the definitions above we could have the following dataset presented in two forms:
Date  Company  Value ($)  Volume (M) 

02/03/20 
AAPL 
470.89 
10.5 
02/03/20 
AMZN 
1981.03 
2.3 
02/03/20 
CLDR 
8.21 
35.1 
03/03/20 
AAPL 
471.03 
11.2 
03/03/20 
AMZN 
1981.10 
1.9 
03/03/20 
CLDR 
8.21 
37.7 
04/03/20 
AAPL 
471.02 
9.4 
04/03/20 
AMZN 
1981.11 
2.2 
04/03/20 
CLDR 
8.36 
29.8 
Date  AAPL Value  AAPL Vol  AMZN Value  AMZN Vol  CLDR Value  CLDR Vol 

02/03/20 
470.89 
10.5 
1981.03 
2.3 
8.21 
35.1 
03/03/20 
471.03 
11.2 
1981.10 
1.9 
8.21 
37.7 
04/03/20 
471.02 
9.4 
1981.11 
2.2 
8.36 
29.8 
Slices of panel data
We can also take different "slices" of panel data too. These produce commonly seen table forms such as timeseries and crosssectional data. They can be thought of as special cases of panel data with only one index (one object and many timepoints for timeseries data, one time point and many objects for crosssectional data). Examples of both of these are shown below:
The index of timeseries data is continuous whilst the index of crosssectional data is categorical. The continuous nature of indices of timeseries data can be thought of imposing adjacency or order to the data. This can add additional complexity to the data as rows can become dependent on each other.
Modelling and generating panel data
The way in which we choose to model the panel datasets will depend on a few key traits:
Timeindex

Total length

Absolute or relative
Objectindex

Total number
Variables

Total number
For example, the panel data above has 2 variables for 3 panel members over 3 dates.
When we do then synthesize new panel data, we can either synthesize values from within the original index domain or extrapolate outside of the original index domain.