Top: variation of the rms of a QPO at different peak frequencies, measured with the various techniques and with and without dead time. Each point represents a simulated QPO with rms = 10% and FWHM = 2 Hz. A total of 281 simulations were used for this plot. (Bottom) Significance measured with each method. The total PDS has about twice the significance of the single-module PDS in the no-dead-time case, as expected, owing to double the number of photons. The CPDS in the no-dead-time case is a factor of higher than the single PDS, and lower than the total PDS by the same amount. The dead-time-affected CPDS, instead, has a much lower level due to the lack of photons. The decrease of significance does not depend on the frequency of the QPO, but only on count rate (see Figure 4).