Example of a variable Kepler star and different period analysis methods applied to the data. Top panel: original (black dots) and 3 h-binned (red line) light curve of the star KIC 7831394. Second panel: global (black) and local (red) auto-correlation functions (ACFs). The best ACF period is found at 26.28 days. Third panel: global wavelet power spectrum (black). The vertical blue line indicates the position of the inflection point. The ratio between the power at the inflection point and the minimum is defined as signal-to-noise ratio (S/N), indicated by the orange arrow between the dotted lines. Bottom panel: gradient of the power spectrum (GPS). The maximum is found at the inflection period P_IP = 4.76 days, marked by the blue line and the red asterisk.