Before revision:  However, because of large human variability both across individuals and within individuals over time, we need to collect calibration (training) data from each individual before each session, which could be laborious and time-consuming, deteriorating its practicality in a real-world context.
After revision:  However, due to notable human variability across individuals and within individuals over time, we need to collect calibration (training) data from each individual before each session, which could be laborious and time-consuming, deteriorating its practicality in a real-world context.