Examples of synthetic flare time series with QPPs are given on the left, and on the right are the corresponding power spectra, where the red solid line is a power law fit, and the red dotted and dashed lines correspond to the 95% and 99% confidence levels respectively. The top two rows show two signals with different background trends, both with a peak above the 99% level in the power spectrum. The bottom two rows show the same signals but with steeper background trends, the result of which is that the peaks no longer reach significant levels in the power spectra.