## 2 Materials and Methods
### Data Analyses  
Recorded DPOAE levels within an individual had to be 6 dB above the noise floor and greater than \(-\)20 dB SPL to be included to provide sufficient monitoring capabilities (refer to review by [2]). As a result, there are data points that do not meet inclusion for clinical utility. Significant DPOAE level changes for the frequency sweep DPOAE paradigms (gross, high-level and mid-level concentrated) were defined as at least a \(\pm\)6 dB absolute change from baseline to following sessions in at least two consecutive frequencies [29].  
Input-output (I/O) functions were created with stimulus level on the x-axis and DPOAE level on the y-axis for the level sweep paradigms. From this function, DPOAE detection thresholds were determined by finding the first level of the varied stimulus tone (L\({}_{1}\) or L\({}_{2}\)) where the DPOAE met the criteria of 6 dB SNR with an overall level greater than \(-\)20 dB SPL and continued growth generally was seen with further increases in the stimulus level. It should be noted that at the highest levels of L\({}_{2}\) varied with a fixed L\({}_{1}\), the DPOAE levels have a tendency to plateau and even decline with further increases in stimulus levels. The stimulus levels typically generating the largest DPOAE levels occur when L\({}_{1}\) is greater than L\({}_{2}\), but when the stimulus levels are equal the DPOAE level can plateau and saturate [31]. To be considered as a significant change, the detection threshold had to change by \(\geq\)9 dB between sessions [30].  
For ratio sweeps, where one frequency is held constant (f\({}_{2}\)) and the other frequency (f\({}_{1}\)) is varied in small step sizes, both the level and phase of the DPOAE can be analyzed. The DPOAE phase can be used to calculate a travel time or group delay for each frequency region tested. Based on the tonotopic organization of the basilar membrane, it is expected that higher frequencies should produce a shorter group delay in comparison to lower frequencies. Ratio sweeps were analyzed in two ways for this study. First, overall DPOAE level across frequency for each ratio within the sweep were analyzed for absolute changes of \(\geq\)6 dB between sessions for a single individual. Second, a customized MATLAB program was used to calculate weighted and unweighted group delays using the phase information from the ratio sweeps. This approach of calculating and comparing weighted and unweighted group delays addressed potential issues that could arise during phase analysis. The MATLAB program plotted the phase of the DPOAE as a function of frequency for the ratio sweeps and used the slope of the phase to calculate a group delay. Weighted group delays took into account the level of the noise floor and favored the data with lower noise floors and greater overall level. Given similar unweighted and weighted group delays, only the weighted group delays were reported here. Criteria for a significant change in group delay for an individual was defined as an absolute change of 0.87 ms or more [31]. In total, 11 analyses for the nine DPOAE paradigms described above were completed (refer to Section 3.4 Summary of Results).