Comparison of methods

Two different approaches to measure the scatter curve are described in the literature. The basic difference between the 'single' method described by [Bruch (1996)] and used successfully in the present study as well as by [Bruch et al. (2000)], and the 'ensemble' method first introduced by [Horne & Stiening (1985)] and later adopted by [Welsh & Wood (1995)] and [Bennie et al. (1996)], is as follows: In the first case the scatter is determined as a function of phase for each light curve of a given system individually. The difference between a smoothed light curve and the original one is taken to define the scatter. The average of many individual curves of this kind then constitutes the mean scatter curve. In the second case the scatter is defined by the deviation of the individual light curves at a given phase from the mean light curve (after trying to account for long term variations). Thus, the final scatter curve resulting from the 'single' method is the mean of the scatter, whereas in the 'ensemble' method it is the scatter around the mean. These are different things!

Welsh et al. (1996) compare the two methods qualitatively. However, a more rigorous comparison has never been made. Moreover, the 'ensemble' method is only described briefly in the references cited above. Therefore, before embarking on the main objective of the present study - the determination of the location of the flickering light source in four systems - a more thorough assessment of the basis of the 'ensemble' method and its relationship to the 'single' method is performed in this section.