A writer for a magazine recently did a test to determine which mid-range digital camera takes the best pictures. Her method is described below. Which part of the method describes an area of potential bias? She gathered 20 of the best-selling cameras and used the same photographer to take 100 pictures with each camera. She ensured that the environment and the subject of each picture were consistent across all cameras and used a 10-point scale to determine picture quality. Of the cameras tested, two were from companies who are major advertisers in the magazine.