## 3 Results and Discussion
### Inter-Individual Differences  
To gain a full insight into how the subjects involved in this experiment perceive \"normal speech level\" on an individual basis rather than as a group, it is imperative to examine and assess the magnitude of the interindividual differences between the subjects. To do so, a visual examination was made by plotting every observation (A-weighted level) made by each subject. The observations collected in the listening part of the experiment are shown on the chart in Figure 3, whereas the ones collected in the reading part of the experiment are shown in Figure 4. For a simpler and clearer comparison, both charts use the same range of the A-weighted levels on the y-axis. Figure 3 shows that the interindividual differences in the listening part of the experiment are considerable, much larger than the ones reported in [34] as the result of a similar experiment using a reproduced speech sample spoken by a single female speaker and reproduced only over headphones. The range of observed values obtained for each  
Figure 4: The range of observations provided by each subject for the A-weighted level of speech in the reading part of the experiment. Figure 3: The range of observations provided by each subject for the A-weighted level of speech in the listening part of the experiment. listener reveals that the perception of what the \"normal\" speech level should be under the described experimental conditions is highly individual. As shown in Figure 3, it is quite easy to find pairs of listeners with completely non-overlapping ranges of observations, e.g., the observations made by one listener never exceed 50 dBA, whereas the ones made by another start at 55 dBA and go considerably higher. The sheer range of observations viewed for the listeners individually is an indicator of their consistency in setting the \"normal\" speech level depending on the listening conditions (reproduction system, speaker voice, visual stimulus). A simple analysis was made, aimed on finding the width of the interval that contains all observations given by a subject by subtracting the highest value of the A-weighted speech level from the lowest one. The results reveal that some listeners (20 percent) can be declared reasonably consistent, i.e., not particularly affected with the changing listening conditions during the listening part of the experiment, as all their observations lie within 10 dB. Most of the listeners (60 percent) are moderately consistent, with all their observations contained in the interval with the width between 10 and 20 dB. Finally, the remaining 20 percent of the listeners are quite inconsistent, with the range of their observations being greater than 20 dB. The extremely wide range of individual observations is also visible at the group level, as illustrated with standard deviations that correspond to individual marginal means shown in Tables 4-6 for each of the three main effects (the choice of the reproduction system, visual stimulus, and speaker voice, respectively) in the listening part of the experiment. As shown, these standard deviations are in the range of 6.4 to 7.8 dB. All the data presented above suggests that the interindividual differences exhibited in the listening part of the experiment definitely have a much stronger influence on the outcome of the experiment than any of the three main effects and their interactions. As a conclusion, any listening test that uses level calibration based simply on setting the \"normal\" level of a calibration speech sample will inherently suffer from a large error that will be built into the observations and the output data obtained from such a test. The observations collected during the reading part of the experiment are shown in Figure 4, and the corresponding descriptive data is presented in Table 8, as shown above.