Galactic spiral structure parameters - Error estimates by numerical experiments

The errors and biases in the galactic spiral-structure parameters estimated from the radial-velocity field of 192 Cepheids are determined by means of numerical experiments. The biases result mainly from the peculiar-velocity dispersion, as well as from errors in the distance moduli and radial velocities. The chief factor affecting the chances of identifying spiral structure from radial-velocity data for Cepheids and supergiants is the accuracy of the distance moduli, and the Cepheid distances now available are quite accurate enough to reveal spiral structure.