Visual Photometry: Colour and Brightness Spacing of Comparison Stars
  A significant amount of data on the historical and current behaviour of variable stars is derived from visual estimates of brightness using a set of comparison stars. To make optimum use of this invaluable collection one must understand the characteristics of visual photometry, which are significantly different from those of electronic or photographic data. Here I show that the dispersion of estimates among observers is very consistent at between 0.2 and 0.3 magnitudes and, surprisingly, has no apparent dependence on the colour of comparison stars or on their spacing in brightness. 
Alan B. Whiting
astro-ph.IM astro-ph.SR