## 4 Discussion
### Interpreting RFF in Clinical Practice  
Regardless of the method used to calculate RFF, it remains that one of the primary challenges hindering the clinical adoption of RFF is a lack of standardized values for typical and disordered voices. Unfortunately, our review of the literature does not provide a clear answer as to which RFF values should be used for a clinically meaningful distinction of vocal health or disorder. On the one hand, we found robust evidence that individuals with voice disorders have consistently lower offset cycle 10 values when directly comparing the range of their reported mean values to those reported in adults with typical voices. Onset cycle 1, however, showed considerable overlap between typical and disordered groups. These findings are surprising since statistical comparisons between those with and without voice disorders were significant in 9 out of 12 studies. This discrepancy could be due to analyses that combined offset and onset cycles rather than reporting offset and onset values in isolation. Thus, at present, there seems to be no clear recommendation for exactly which cycle, or combination of cycles, would effectively distinguish between typical and disordered voices. This area of research is still ongoing, with recent studies looking to enhance the sensitivity of RFF via advancing computational methods (e.g., automated, rule-based decision criteria [51]) as well as combining RFF with other acoustic parameters to improve the accuracy of RFF as a clinical tool of diagnosis [63]. Importantly, offset cycle 10 and onset cycle 1 ranges reported in this review were created without consideration of the number of subjects enrolled across studies, leading to an equal weighting for studies that enrolled few subjects and those that enrolled many. Instead, a weighted average would be the best way to help establish normative rangesand values across healthy and disordered populations. At present, the number of studies reporting RFF values for adults with typical voices (\(n=14\)) may be sufficient to establish an acceptable normative range, but more studies need to report raw RFF values (\(M\), SD, range) for aging adults and children with typical voices to establish normative ranges across the lifespan. At present, there seems to be stronger evidence for the utility of RFF as a clinical tracking tool for within-subject changes over several voice productions as well as for quantifying progress pre- and post-therapy. Tracking of physiological parameters (kinematic stiffness, aerodynamics) were shown to be moderate-to-strongly correlated with RFF for individual analyses of vocally healthy speakers varying their vocal effort and strain [1, 7]. Furthermore, [PERSON] et al. [47] and [PERSON] et al. [24] reported improvements in onset cycle 1 values (i.e., more positive values) in patients with tension-based voice disorders following vocal therapy. Despite the promise of these results, there were few studies that examined RFF as a clinical outcome measure [13, 24, 36, 47]. Indeed, a systematic review with meta-analysis is considered the highest level of evidence for understanding treatment efficacy and establishing clinical guidelines [64]. However, different intervention approaches (single therapy visit, multiple therapy visits, surgery) and various treatment groups (e.g., MTD, VH, polyps, typical) across the studies described in our review, make a meta-analysis impossible to undertake at this time. We recommend that future treatment studies of patients with tension-based voice disorders incorporate RFF into their standard voice protocols to increase the number of studies available for a large-scale analysis and assist in validating RFF as a treatment outcome. This would not only provide evidence for RFF as an appropriate outcome measure in specific patient populations, but also help to define what a clinically meaningful change in RFF may be over the course of intervention.