Distributions of the fractions of variable and non-variable magnetic white dwarfs with M ≤ 1 M_⊙. The blue solid line represents the number of non-variable stars younger than 2 Gyr divided by the total number of stars younger than 2 Gyr with an average field strength lower than the value in the x-axis. The red solid line refers to the same quantity for magnetic white dwarfs older than 2 Gyr. Dotted lines show the estimate of the uncertainties.