def solar():
    """
    Solar panel example from Box, Hunter and Hunter, 2nd edition, Chapter 5,
    page 230.


    Description
    ------------
    The data are from a solar panel simulation case study.

    The original source that Box, Hunter and Hunter used is
    https://www.sciencedirect.com/science/article/abs/pii/0038092X67900515

    A theoretical model for a commercial system was made. A 2^4 factorial
    design was used (center point is not included in this dataset).

    The factors are dimensionless groups
    (https://en.wikipedia.org/wiki/Dimensionless_quantity), related to:

    A = total daily insolation,
    B = the tank capacity,
    C = the water flow through the absorber,
    D = solar intermittency coming in.

    All 4 factors are coded as -1 for the low level, and +1 for the high lever.

    The responses variables are
    y1: collection efficiency, and
    y2: the energy delivery efficiency.

    Dimensions
    ----------
    A data frame containing 16 observations of 6 variables (A, B, C, D, with
    y1 and y2 as responses.)

    Source
    ------
    Box, G. E. P. and Hunter, J. S. and Hunter, W. G., Statistics for
    Experimenters, 2nd edition, Wiley, Chapter 5, page 230.

    Example
    -------

    """
    pass
