In a research study that compares patterns of sleep detected by a Fitbit (all participants) to patterns of sleep detected by two different brands of

In a research study that compares patterns of sleep detected by a Fitbit (all participants) to patterns of sleep detected by two different brands of actigraphs (participants randomly assigned to one of the two brands) and to the gold standard of polysomnography (all participants) and in which the two brands of actigraphs are compared with each other and to polysomnography. Which variables would be considered covariate in the use of ANCOVA