After and/or during a data collector training, data collectors are assessed through surveys, quizzes and/or observations to ensure that the protocols are being followed as designed.
This is important so that all data collectors use the same methodology in asking questions and/or observing behaviors or the environment so that data can aggregated and compared. This reduces any variability or bias that could be caused by the data collector and is not an accurate depiction of the population of interest.
Inter-rater reliability is the degree to which different raters/observers give consistent estimates of the same phenomenon. The goal is to reduce variability or errors caused by data collectors.