**Definition:** A `Series`

is the programmatic embodiement of a **data line**, i.e. a **labelled sequence of data points**:

If the labels $i_1,\dots, i_n$ correspond to

*unique ID of individuals from a population*$\Omega=\{1,\dots,N\}$, and the data points correspond to the*value of some characteristic*$X:\Omega\rightarrow \mathbb R$ for these individuals, the data line is called a**population sample**(of observations): $x_{i_l} = X(i_l).$

*instant in times*$i_1 < i_2 <\cdots < i_n$ and the data points correspond to the value of the characteristic $X$ of a single fixed individual $\omega\in \Omega$ but at different moment in times, the data line is called a**time series**: $x_{i_l} = X_{t=i_l}(\omega).$

**Definition:** A Data Frame is the programmatic embodiement of a **data table**, i.e., a 2D array of values with rows and columns explicitely **labelled**. Usually,

- the
*row labels*, say $i_1,\dots, i_n$ correspond to a sample of the unique individual IDs in a population $\Omega = \{1,\dots, N\}$

- the
*column labels*correspond to the name $X,Y,Z$ of some characteristics $X,Y,Z:\Omega\rightarrow\mathbb R$ of our population $\Omega$

- The actual
*values*in the data tables are the values of these characteristics for the sampled individuals.

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