I understand $y_{ij}$ can be used to represent a cell in a matrix (i.e., the value in row $i$ and column $j$), particularly where the length of $j$ is equal for all $i$. I know it can also be used to index a vector $y_{i.}$ where each element is a vector indexed by $j$, and that this can permit different lengths of $j$ for each $i$.
- What is the general name of this mathematical structure?
- Should I think of it as a matrix, a vector of vectors, a long data frame (with one column for each index and one column for the value) or something else?