Consider a regression model with time series data measured at quarterly intervals and let the variables $X_{1t}$, $X_{2t}$, and $X_{3t}$ be
$X_{1t} = \begin{cases} 1, \text{ if observation t is in the first quarter} \\ 0, \text{ otherwise} \end{cases}$
$X_{2t} = \begin{cases} 1, \text{ if observation t is in the second quarter} \\ 0, \text{ otherwise} \end{cases}$
$X_{3t} = \begin{cases} 1, \text{ if observation t is in the third quarter} \\ 0, \text{ otherwise} \end{cases}$
The variables $X_{1t}$, $X_{2t}$, and $X_{3t}$ are examples of:
robotic variables
dummy variables
time-consistent variables
homoskedastic variables
pneumatic variables
