## 2 Materials and Methods
### VHI Questionnaire  
Voice treatment was delivered through a combination of direct and indirect techniques. Sessions were scheduled weekly with the number of sessions varying among patients, ranging from one to eight and averaging four sessions in total, with each session lasting approximately 40 min. Patients self-assessed their vocal condition using the VHI questionnaire prior to and following treatment [19; 20; 21; 22; 23].  
The VHI, a psychometric assessment tool widely used for evaluating voice disorders, was developed by [PERSON] et al. in 1997 [12] and was designed to be broadly applicable to adult patients with voice disorders, regardless of the type, including those who have undergone laryngectomy. In this study, the Korean translation of the VHI was used. The tool comprises 30 questions, which are equally allocated across three domains: functional (F), physical (P), and emotional (E). Each variable is scored on a five-point scale (0-4), representing \"never\", \"almost never\", \"sometimes\", \"almost always\", or \"always\". The scores are expressed in subscores (0-40) and a total score (0-120). Firstly, the functional subscale is used to evaluate the difficulties in communication caused by voice problems and assess the impacts of these issues on social interactions. Secondly, the physical subscale is used to measure the physical challenges and discomfort experienced during the vocalization process. Lastly, the emotional subscale is used to assess the psychological effects of voice problems, such as emotional stress and reduced self-esteem. Higher scores indicate greater severity of the disorder [24; 25; 26; 27; 28]. The details of the questionnaire are shown in Table 2.  
\begin{table}
\begin{tabular}{c c} \hline
**Number of Samples** & **66 Female and 36 Male Voices** \\ \hline Average age & 51 \\  & vocal fold polyp (31), vocal paresis (2), intracordal cyst (2), vocal nodule (19), mutational dysphonia (1), thyroid nodule (1), hoarseness (1), muscle tension \\ Types of voice disorders (numbers) & _dysphonia (7), sulcus vocalis (2),_ presbyphonia (8), dysphonia (10), vocal cyst (7), leukoplakia (2), vocal palsy (1), thyroid cancer (2), vallecular cyst (1), vocal mass (2), laryngopharyngeal reflux (1), none (3) \\ Number of responsive samples (effectiveness, +) & 93 (total AHI score), 78 (functional subscale score), 73 (emotional subscale score) \\  & gender, smoking status, alcohol consumption, voice \\ Variables & user status, coffee consumption, comorbidity, \\  & surgery status, voice training status \\ \hline \end{tabular}
\end{table}
Table 1: Overview of the dataset utilized in this study.  
Binomial logistic regression analysis is a statistical technique applied when the dependent variable is categorical, particularly dichotomous, with only two possible outcomes [29]; this method differs from linear regression in its utilization of the logit function to model data, thereby enabling the prediction of the probability of a specific class or event through fitting the data to a logistic curve [30]. The logistic regression model is used to calculate the log odds of the dependent variable, which corresponds to the likelihood of an event occurring, in order to estimate the probability of a binary event's occurrence based on one or more predictor variables. Logistic regression is commonly utilized for maximum likelihood estimation to estimate the model parameters and evaluate the influences of independent variables on the dependent variable; this method is widely used to understand the effects of particular variables on an outcome and to make predictions for future observations [29; 30]. In this study, a binomial logistic regression model was fitted to analyze eight parameters as predictors of voice therapy based on the VHI. The dependent variable is VHI value (\(+\) and \(-\)), while the eight independent variables are gender, smoking status, alcohol consumption, voice user status, coffee consumption, comorbidity, surgery status, and voice training status.