Anexo 2 Relato de una de las madres de los adolescentes entrevistados

To explore the psychological factors that best predicted adherence and/or non- adherence to CPAP use, a stepwise DA was conducted. Predictive DA addresses the question of how to assign cases to groups. The DA function uses an individual’s scores on the predictor variables to predict the category in which the individual belongs. In the present study, patients were classified a priori into two groups, adherent and non-adherent, determined by completion of the diagnostic and

implementation phases of the study (i.e., patients who only completed the diagnostic phase were categorised as non-adherent, and patients who completed both diagnostic and implementation phases were categorised as adherent).

A stepwise estimation procedure was used to predict group membership in either the adherent or non-adherent categories and explore the impact of the following psychological factors: mood, personality, self-efficacy, locus of control, and health belief. The stepwise estimation procedure was employed. The stepwise procedure involves entering the psychological factors into the discriminant function one at a time on the basis of their discriminating power. Eventually, either all independent variables will have been included in the function or the excluded variables will have been judged as not contributing significantly to further discrimination.

Assumption testing determined the appropriateness of DA and included tests for normality, non-multicollinearity, and the absence of outliers. It must be noted that the assumption of homogeneity of variances/co-variances was not supported;

however, given that the multivariate Box’s M test for homogeneity is particularly sensitive, the stepwise DA procedure was still carried out (Tabachnick & Fidell, 2006). Initial results of the stepwise DA procedure are shown in Table 17.

Table 17

Stepwise Statistics Loadings Ranked According to Relative Importance of the Predictors for CPAP Adherence and Non-Adherence

Entered Function p

Anger/Hostility .624 .0005

Vigour/Activity .530 .0005

Self-Efficacy .491 .0005

Internal Health Locus of Control .465 .0005

Depression/Dejection .424 .0005

Perceived Benefits .412 .0005

Statistically significant predictor variables entered into the model ranked according to their discriminating power included anger/hostility, self-efficacy, internal health locus of control, perceived susceptibility, depression/dejection, and perceived benefits. Significant mean differences were observed for all identified predictors. The discriminant function revealed a significant association between groups and all predictors, accounting for 58.8% of between-group variability. The absence of the remaining psychological factors meant that they provided no statistically significant loading on the discriminant function, and they were therefore deemed to have poor predictive value and were excluded from the model. The cross-validated classification showed that overall, 87.8% of patients were correctly classified, with predictions for the non-adherent group (92%) being more accurate than those for the adherent group (82.6%). In order to determine if discrimination was clinically significant as well as statistically significant, that is, the ability of the discriminate function to correctly classify those that are CPAP adherent a ROC curve was also constructed and displayed in Figure 10.

Figure 10. ROC curve differentiating clinically significant psychological predictors of CPAP adherence.

Figure 11 provides a graphical representation of the sensitivity and specificity for clinical significance. The accuracy is expressed as the area under the ROC curve (AUC) and provides a useful parameter for comparing the statistically significant psychological factors that predict CPAP adherence from the DA. The results from the ROC curve showed strong clinical significance associated with the psychological factors depression/dejection (AUC = .852), anger/hostility (AUC = .862), and vigor/activity (AUC = .724); moderate clinical significance was associated with the psychological factors for internal locus of control and perceived benefits, while weak clinical significant results are exhibited for the psychological factors self-efficacy and perceived susceptibility.

To further explore mean difference at the diagnostic phase amongst the identified psychological predictors from the stepwise DA procedure, a MANOVA was conducted. The MANOVA procedure highlights any specific mean differences between the CPAP-adherent and non-adherent groups in relation to the identified predictors. Assumptions testing mirrored those from the DA and the outcome of the MANOVA is shown in Table 18.

Table 18

Means and Standard Deviations for Identified Predictor Variables for the Adherent (n = 69) and Non-Adherent Groups (n = 87)

Adherent Non-adherent p

M (SD) M (SD)

Anger/Hostility 13.27 (5.49) 21.87 (5.57) .0005

Vigour/Activity 14.04 (4.71) 19.35 (6.08) .0005

Self-Efficacy 29.79 (5.24) 25.59 (3.59) .0005

Internal Health Locus of Control 25.49 (6.41) 29.45 (4.75) .0005 Perceived Susceptibility 15.43 (2.03) 14.66 (1.23) .004 Depression/Dejection 14.24 (6.39) 23.49 (6.97) .0005 Perceived Benefits 11.36 (1.94) 12.11 (1.32) .005

Results from the MANOVA yielded a statistically significant difference amongst the identified predictive psychological factors based on a patient’s group membership to the CPAP-adherent or non-adherent group (F(7, 148) = 30.166, p = .0005; Wilk's Λ _{= 0.412, partial η2 = .58). Specifically, means and ANOVA}

(conducted for comparative purposes) results showed that the greatest mean difference is noted on the mood sub-scales anger/hostility (F(1,68) = 92.746, p = .0005) and depression/dejection (F(1,68) = 72.742, p = .0005). This result suggests that patients who were identified as non-adherent reported more symptoms associated with these mood states than the adherent group. The remaining psychological

factors—vigour/activity (F(1,68) = 35.628, p = .0005), self-efficacy (F(1,68) = 35.100, p = .0005), internal health locus of control (F(1,68) = 19.604, p = .0005), perceived susceptibility (F(1,68) = 8.409, p = .004), and perceived benefits (F(1,68) = 8.086, p = .005)—showed significant yet small mean differences between groups. Interestingly, non-adherent patients at the diagnostic phase reported more

vigour/activity, a greater sense of internality, and slightly better understanding of the perceived benefits associated with CPAP use to treat OSA. The above results are further discussed in greater detail in the Discussion.

CHAPTER 4