Question

Scientists hypothesized that finer grain iron oxide on video and audio tapes would result in better recording and playback. They produced ten tapes with a range of partial sizes, recorded them, and tested the quality of the playback. Electronic instruments and human visual and audio reaction were used to determine whether there was any significant difference between the tapes with different particle sizes of iron oxide

What is the constant variable? Because I think there's none but I'm not sure if I'm right