Left panels: probability distribution of observing a rotation amplitude (A_obs) given a true value of the amplitude (A_true) and a uniform distribution of inclination angles (i). Different tones of gray indicate different probability levels, from 8% (darkest gray) to 1% (lightest gray), in steps of 1%; white regions corresponds to probability below 1%. The continuous line is the A_obs = A_true relation. In the upper panel A_true values are drawn from a uniform distribution, in the lower panel they are drawn from a Gaussian distribution. The probability is estimated in square bins with sides of 1.0 km s^-1 spaced by 0.25 km s^-1 in both directions of the plane. Right panel: distribution of sin(i) for a uniform distribution of i.