How Does Day-to-Day Variability in Psychophysiological Data Affect Classifier Accuracy?

Combined psychophysiological measures have been used to determine mental workload in operators, but the day-to-day reliability of these measures has not been determined. Data were collected four times over a one month period. Two classifiers were trained with these data and their ability to correctly discriminate between two levels of task difficulty with new data was tested. Both classifiers very accurately discriminated between the two levels of task difficulty using data collected on the same day as the training data. However, the accuracy was considerably reduced when tested on data from days different from the training data. The implications for application of these procedures are discussed.