Download Blink behaviour based drowsiness detection

12 Discussion
12.1 Model for boundaries
The linear model chosen for the boundaries assumed that the variables changed in a linear
way over the drowsiness stages, without inter-individual differences. It was not possible to set
up model considering individual differences, as the boundaries should be based on
information derived only from the first five minutes of the alert drive.
The program identified some participants as more drowsy and some as less drowsy compared
to the references. This would imply an existence of inter-individual differences in the
development of drowsiness, which made it difficult to find boundaries connected to blink
related measures that were valid for all participants. The pattern seemed to be consistent
within the individuals though; the program rating was either consequently higher or
consequently lower than the reference measure for one individual. Comparing the program
diagnoses to five minute intervals with incidents or accidents also indicated individual
differences as the program diagnose sometimes was three when it should be four.
When adjusting boundaries for the KSS based model it was also found that it was hard to get
good correspondence for both alert and drowsy drivers. A possible explanation could be that
the relationship between the variables and KSS was not as predicted linear. The model was
based on figures over the relationship, but they contained few data points since the KSS
ratings had been made only in five minute intervals. Perhaps KSS measured something else
than what was measured by changes in blink behaviour. KSS could indicate earlier signs of
drowsiness than the blink parameters. Another explanation could be that the participants had
reached the last steps of KSS too soon in the drowsy condition. Many participants were at the
last KSS steps already in the beginning of the drowsy drive. KSS is an absolute rating scale
but the participants may still have used it as a relative scale and when reaching the last steps
realizing that they could get even drowsier. Another possibility could be that they were
reporting boredom instead of drowsiness.
12.2 Conversion of KSS and OSS to drowsiness stages
Both reference measures had to be converted to the drowsiness scale to make a comparison
possible. KSS had steps with verbal descriptions, which made the conversion easier. Still it is
not sure that the drowsiness reported by the driver corresponded to the physiological
drowsiness level indicated by the program. The reported level was a subjective choice made
by the driver and it is likely that individual differences also existed in this choice.
Converting OSS to the drowsiness stages of the program was more difficult as the steps had
no verbal description of drowsiness level. It was hard to know if drowsiness scale and OSS
covered the same physiological states and in which way they could be compared. The increase
of duration already at step one of the OSS scale though indicated that this step corresponded
to drowsiness stage three. Many participants had got OSS zero even though changes in blink
behaviour were found, which complicated the comparison. It could be believed that the
drowsiness scale was more sensitive than OSS and could find earlier signs of drowsiness or
that the two scales measured different physiological processes.
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