## 3 Results
### Between- and Within-Subject Analyses  
A total of 12 studies completed formalized comparisons between experimental and control groups, with 9 studies finding statistically significant differences. The strongest evidence was found in comparisons involving adults with typical voices and subjects with VH, wherein 100% of studies (\(n=4\)) reported significantly lower RFF values for those with VH. However, discrepancies arose when trying to pinpoint whether offset or onset cycles were different. One study reported significantly lower offsets [27], one reported significantly lower onsets [47], and the remaining two studies reported lower RFF values across both offsets and onsets [6; 13]. Across other studies, there was consistent evidence  
Figure 3: Boxplot of average RFF offset cycle 10 and onset cycle 1 values in studies on adults aged 18–65 years with typical voices. that patients with PD and those with adductor laryngeal dystonia had significantly lower RFF values as a whole compared to those with typical voices [4; 5; 49]. Other studies examined the impact of RFF across the lifespan. One study compared children who do and do not stutter [30] and another compared children with and without vocal fold nodules [35]; neither found differences between groups for any RFF measure. However, [PERSON] et al. [35] reported that RFF onset cycle 1 was lower in younger children compared to older children, pointing to a potential maturational effect for revoicing. A comparison undertaken between young children (age 4), older children (age 8), and young adults showed similar results, wherein young children had lower onset cycles (1 and 2) compared to young adults [46]. A study by [PERSON] et al. [44] examined differences between young adults and aging adults, finding no differences in RFF values; however, a study by [PERSON] [12] did find significant differences for onset values only. Within-subject analyses were undertaken to understand RFF sensitivity to subject fatigue. Four studies examined how vocal load impacted RFF [32; 34; 36; 43], including short-term (a few hours) and long-term (multiple weeks). Findings were mixed: pre- to post-fatigue comparisons showed that offset cycle 10 may assist in tracking fatigue over the workday [43] but onset cycle 1 seemed to be more sensitive to long-term tracking over several weeks [34]. [PERSON] et al. [32] saw no impact of a lab-based vocal loading task on offset cycle 10 or onset cycle 1, whereas [PERSON] and [PERSON] [36] reported composite changes in both measures following a formalized loading paradigm. The relationship between the proposed physiological mechanisms of RFF (i.e., kinematics, laryngeal muscle tension, and aerodynamics) and RFF offset cycle 10 and onset cycle 1 were examined via kinematic estimates of laryngeal stiffness [1; 44], an aerodynamic ratio of vocal intensity to estimated subglottal pressure [7], and vocal fold ab/adduction degree and timing [44]. To create within-subject variation, speakers with typical voices modulated vocal effort and/or strain across several voice productions. Group analyses yielded consistently poor-to-moderate relationships between RFF and the physiological parameters of laryngeal stiffness, aerodynamic ratio, and both kinematic degree and timing. Importantly, however, within-subject analyses demonstrated a marked improvement in the strength of these relationships, ranging from moderate to strong [1; 7]. Accordingly, the authors supported the use of RFF as a clinical tracking tool to monitor an individual's voice changes over a course of therapeutic intervention (where it was assumed that vocal effort and/or strain would decrease over time). Four studies specifically investigated how RFF may change over the course of various interventions. Two studies examined RFF changes over the course of vocal therapy, including a single-session treatment paradigm as well as a full course involving several therapy sessions. [PERSON] et al.