A calibration apparatus is disclosed herein. An example includes a printing mechanism, a media control mechanism, a sensor, and a processor. The processor controls the printing mechanism to print a first pattern of first elements on a print medium and a second pattern of second elements on the print medium that are interleaved with the first elements. The processor also actuates the media control mechanism to advance the print medium, determines a centroid of at least one of the first elements based on a first measured reflectance by the sensor, and determines a centroid of at least one of the second elements based on a second measured reflectance by the sensor. The processor also determines a print medium advance error based on the at least one determined centroid of the first elements and the at least one determined centroid of the interleaved second elements.