What are the statistics used to compare differences between baseline and week 2, baseline and week 4, and week 2 and week 4? Why does the collection at 2 weeks and not just at the end of the intervention at 4 weeks?

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To date, there has been little research on the impact of mindfulness and acceptancebased interventions on resilience, defined as the "personal qualities that enable one to thrive in the face of adver sity"1, p. 75. However, considering available evidence that resilience can mitigate the risk factors associated with posttraumatic stress disorder (PTSD),2,3 in this paper, we argue that resilience should obligatorily be investigated in these types of interventions. The most commonly used mindfulness and acceptancebased interventions available for individuals with PTSD are based on the principles of acceptance and commitment therapy (ACT), and many studies have documented the positive impact of such interventions on the improvement of PTSD symptoms.47 However, few of these studies have focused on the impact of ACTbased interventions on the resilience of individuals with PTSD. Overall, ACTbased interventions for PTSD aim to minimize a person's engagement in avoidance coping behaviors (e.g., thought suppression and substance use) that aggravate posttraumatic stress responses and psychological distress.8 More specifically, a major goal of ACT is to reduce experiential avoidance, or the unwillingness to contact the present moment and experience unacceptable emotions, thoughts, and sensations.9 In experiential avoidance, a person alters the form and frequency of distressing internal experiences, such as unwanted, intrusive thoughts, feelings, and bodily sensations.9 Paradoxically, however, along with temporarily deescalating post traumatic stress responses by promoting avoidance of internal and external stimuli associated with the traumatic event, experiential avoidance can also result in greater psychological distress and emotional dysregulation, which leads to exacerbation of PTSD symptoms.10,11 Accordingly, by encouraging individuals to practice acceptance of the present moment without defending or changing distressing and unwanted internal experiences,9 ACTbased inter ventions teach individuals to develop mindfulness (i.e., "paying at tention in a particular way: on purpose, in the present moment, and nonjudgmentally"12, p. 4) and acceptance (i.e., "experiencing events fully and without defense, as they are"13, p. 32). While there is abundant evidence that ACTbased interventions result in improved resilience in the general population,1417 previous research on the impact of ACTbased interventions on resilience did not include individuals with PTSD. Yet, based on previous findings that resilience has protective properties to mitigate the negative effects of trauma and PTSD,1820 it can theoretically be predicted that ACTbased interventions would enhance the resilience of such individuals.21 Another gap in previous literature is that very few previous studies explored the impact of ACTbased interventions on mind fulness of individuals with PTSD, and the results of these few available studies are inconsistent. For instance, while Burke et al.,22 and Morton et al.,23 observed the positive impact of ACTbased in terventions on mindfulness of patients with PTSD, Wharton et al.24 reported that only one of the five aspects of mindfulness (i.e., non reactivity) significantly improved after an ACTbased intervention. Similarly, previous findings about the effects of ACTbased inter ventions on experiential avoidance in PTSD are also inconsistent. Specifically, while several singlearm ACT intervention studies found significant improvement in experiential avoidance and PTSD at postintervention,5,6 other studies failed to replicate this pattern.2527 Seeking to fill the aforementioned gaps in the literature, in this study, we examined the relationships between resilience, mind fulness, experiential avoidance, and PTSD symptoms after the use of a mindfulness and acceptancebased intervention. The results of the initial assessment of efficacy and acceptability of our ACTbased mobile app were reported elsewhere.28 However, Reyes et al.28 did not explore the relationships between the variables under in vestigation here. In this context, the present study aimed to de termine the relationships between resilience, mindfulness, experiential avoidance, and PTSD in the use of an ACTbased mobile app for college student veterans with PTSD symptoms. The intervention was delivered through a smartphone app. This decision was underpinned by several considerations. First, delivering the intervention through an app was deemed relevant because our study population (i.e., college student veterans with PTSD symptoms) has highly stigmatized views of traditionallydelivered mental health services.29,30 Second, our appbased intervention was very accessible to the participants, as smartphone apps are ubiquitously used by college students.31 Third, delivering our ACTbased intervention through an app also contributed to the novelty of our approach. In fact, although appbased ACT interventions have frequently been tested as selfmanagement tools for depression and anxiety,3235 very few available ACTbased mobile apps target specifically PTSD symptoms,36 and none of the previous studies explored the re lationships among PTSD symptom severity, resilience, mindfulness, and experiential avoidance using appbased ACT interventions.

2 | METHODS 2.1 | Design The present study used a quasiexperimental pre-posttest, single group study design.

2.2 | Ethical approval and conduct Ethical approval to conduct this study was obtained from the re search ethics board of the participants' university. Participation in the study was confidential and voluntary. The participants were in formed about their right to withdraw from the study at any time; they were also assured that their personal identifying information would not be published. Informed consent was obtained before the participants started using the appbased intervention and data collection.

2.3 | Participants Study participants were recruited through a recruiting email that contained all essential information about the study. Inclusion criteria were as follows: (a) age at least 18 years old or older; (b) a U.S. military veteran; (c) a current undergraduate/graduate student at a U.S. university; (d) history of traumatic exposure based on the Pri mary Care PTSD Screen for DSM5 (PCPTSD537); (e) a score of 33 (i.e., the cutoff score for probable PTSD) or higher on the PTSD Checklist for DSM5 (PCL538); and (f) currently owning an iOS or Android smartphone. Interested individuals who responded to the recruiting email were provided with a Qualtrics link containing PTSD screening surveys (i.e., PCPTSD5 and PCL5). A total of 23 college student military veterans met all inclusion criteria and participated in the study.

2.4 | Procedures Eligible participants were invited to attend a oneonone, inperson orientation to the study. During the orientation, the first author in troduced the principles of mindfulness and ACT, explained how to download and navigate the app, as well as provided instructions on the minimum daily and weekly requirements of using the app. The study participants were encouraged to (a) listen daily to at least one of the seven audioguided mindfulness meditations embedded in the app and (b) watch the weekly ACT videos sent to their app and write a reflection about the video within the app. During the orientation phase, the participants also completed baseline measures of resilience, mindfulness, experiential avoidance, and PTSD. The participants started using the app immediately after the orientation. The smartphoneapp intervention was delivered through the Me tricWire online hosting platform, which is compliant with U.S. laws related to data privacy. The participants received daily push notifi cations to remind them to use the app, and the research team was able to track the app usage of each participant (i.e., the type of ACT exercises used and the time they started and finished doing the exercises). Participation in the intervention lasted four weeks. At the end of Week 2 and 4 of the app use, the participants completed measures of resilience, mindfulness, experiential avoidance, and PTSD embedded within the app. Further detail of the intervention is provided elsewhere.

2.5 | Measures 2.5.1 | Connor-Davidson Resilience Scale The Connor-Davidson Resilience Scale (CDRISC)1 is a 25item in ventory that measures the resilience level. Each item is rated on a 5point scale (from 0="not true at all" to 4="true nearly all the time"). The CDRISC items include various statements about char acteristics of resilience (e.g., "I am able to adapt to change"; "I can handle unpleasant feelings"). The sum scores range from 0 to 100, with higher scores indicating a higher level of resilience. The CDRISC was reported to have good internal consistency (=0.89) and test-retest reliability (intraclass correlation coefficient: 0.871).

2.5.2 | Mindful Attention Awareness Scale The Mindful Attention Awareness Scale (MAAS)39 is a 15item self report questionnaire that measures an individual's dispositional mindfulness characterized by "open or receptive awareness of and attention" to the present moment39, p. 822. Each item of the MAAS is rated on a 6point scale (from 1="almost always" to 6="almost never"). The MAAS items include different statements about every day experiences (e.g., "I drive places on 'automatic pilot' and then wonder why I went there"; "I snack without being aware that I'm eating"). The sum scores range from 15 to 90, with higher scores indicating higher levels of mindfulness. The MAAS was previously reported to have good internal consistency (=0.82) and test-retest reliability (intraclass correlation coefficient: 0.8139).

2.5.3 | Acceptance and Action QuestionnaireII The Acceptance and Action QuestionnaireII (AAQII)40 is a 7item questionnaire that measures experiential avoidance or unwillingness to experience unacceptable emotions and thoughts. Each item is rated on a 7point scale (from 1="never true" to 7="always true"). The AAQII includes different statements about emotional control and avoidance (e.g., "I'm afraid of my feelings"; "I worry about not being able to control my worries and feelings"). The sum scores range from 7 to 49, with higher scores indicating a greater experiential avoidance. The AAQII was reported to have good internal con sistency (0.84) and test-retest reliability at 3 (0.81) and 6 (0.79) months.40

2.5.4 | PTSD Checklist for DSM5 The PTSD Checklist for DSM5 (PCL5)38 is a 20item selfreport scale that evaluates the severity of PTSD symptoms. Each item is rated on a 5point scale (from 0="not at all" to 4="extremely")of PTSD symptoms experienced within the last month. The sum scores range from 0 to 80, with higher scores indicating greater severity of PTSDsymptoms. A cutpoint total score of 33 is generally considered to be a reasonable value for a provisional diagnosis of PTSD among a nonclinical, community sample.38 The PCL5 was found to have good internal consistency ( = 0.94) and test-retest reliability (r = 0.8241).

2.6 | Data collection Demographic data and baseline measures (i.e., resilience, mind fulness, experiential avoidance, PTSD symptom severity) were col lected via a paperandpencil survey during the study orientation phase. Midintervention (Week 2 of the intervention) and post intervention (Week 4) measures of resilience, mindfulness, experi ential avoidance, and PTSD symptom severity were collected via the MetricWire smartphone app. The MetricWire hosting platform was also used to quantify the days when the study participants meditated (i.e., performed the mindfulness exercises) and the number of mindfulness exercises they used during the intervention period.

2.7 | Data analysis SPSS (Version 25) and SAS (Version 9.4) were used for data analysis. Descriptive analysis was performed to explore the de mographic characteristics of the participants, as well as to com pare their scores of resilience, mindfulness, experiential avoidance, and PTSD symptom severity at baseline (Time 1), end ofWeek2 (Time 2), and endofWeek4 (Time 3). To determine if there were significant improvements in all of the outcomes, we calculated the differences in the measurements between Time 2 and Time 1 (ImprovementT2=ScoreT2ScoreT1) and between Time 3 and Time 1 (ImprovementT3=ScoreT3ScoreT1) foreach participant. Thereafter, ttests were run to determine the im provement of scores by comparing the participants' scores be tween Time 1 and Time 2 and between Time 1 and Time 3 (see Table 1). Pearson's correlations were calculated to determine the association between improvements of the scores between Time 1

While no significant correlations were found in the changes in the four outcome variables between Time 1 and Time 2 (Table 2), the analysis yielded significant correlations between changes in resi lience, mindfulness, and PTSD symptom severity between Time 1 and Time 3 (Table 3). Specifically, the increase in mindfulness between Time 1 and Time 3 led to the improvement in resilience (r=0.478, p=0.021). Furthermore, changed scores between mindfulness and PTSD symptom severity were significantly negatively correlated; hence, the improvement in mindfulness was significantly correlated with the improvement in PTSD between Time 1 and Time 3 (r = 0.632,

DISCUSSION

To the best of our knowledge, this study is the first to explore the relationships between resilience, mindfulness, experiential avoid ance, and PTSD symptom severity among individuals using an ACTbased mobile app intervention. After using the mobile app for 4 weeks, the participants demonstrated improvements in resilience levels that, in turn, were found to be significantly related to im provements in mindfulness. Although none of the previous studies has particularly explored the relationships between improvements in resilience and mindfulness among individuals with PTSD symptoms who participated in an ACTbased mobile app intervention, our re sults are consistent with previous research on mindfulnessbased interventions. For instance, several studies using mindfulnessbased interventions (but not specific to ACT) demonstrated a significant relationship between improvements in resilience and mind fulness.42,43 Based on this evidence, we conclude that the relation ship between improvements in resilience and mindfulness is related to the improvement of emotion regulationa construct considered to be a process of both resilience and mindfulness.44 Emotion reg ulation, defined as the ability to modulate one's emotional responses to situations,45 can be developed through mindfulness processes, such as acknowledging the presence of negative internal experiences and experiencing these unwanted emotions and thoughts, rather than avoiding or attempting to change them.46 Consequently, an individual's ability to effectively regulate one's emotions particularly, the distressing onesprovides an alternative to pro blematic ways of coping (i.e., avoidance and suppression) and limits reactivity to anxietyproducing emotions and thoughts.47,48 Furthermore, our results on the significant relationship between improvements in mindfulness and PTSD symptom severity are con sistent with the findings of a mindfulnessbased stress reduc tion intervention for military veterans49 as well as from a longitudinal descriptive study which demonstrated that mindfulness significantly predicted PTSD recovery.50 Moreover, our findings on the significant relationship between improvements in mindfulness and PTSD symptom severity also contribute to previous research on ACTbased interventions for individuals with PTSD. As mentioned previously, available research on the effect of ACTbased interven tions on the relationship between improvements in mindfulness and PTSD is scarce. Specifically, while several studies reported im provements in both mindfulness level and PTSD symptom severity among the participants who used an ACTbased intervention,22,24 these studies did not investigate the relationship between im provements of the two variables. Therefore, our finding on the sig nificant relationship between increased mindfulness and reduced PTSD severity provides additional empirical evidence about the mechanisms of action of ACTbased interventions on mindfulness of individuals with PTSD. Taken together, the results of the present study extend previous research by exploring the roles of resilience, mindfulness, and ex periential avoidance in reducing PTSD symptom severity through the use of a mindfulness and acceptancebased intervention. Based on our findings about the significant positive relationship between im provements in mindfulness and resilience, the significant negative relationship between improvement in mindfulness and PTSD symp tom severity, and the negative (although not significant) relationship between improvement in mindfulness and experiential avoidance, we can conclude that resilience is more of a consequential process than an antecedent construct to the mindfulness and acceptance pathway. Specifically, our findings on the significant relationship between im provements in resilience and mindfulness offer empirical evidence to Thompson et al's.21 theoretical postulation of multiple pathways through which mindfulness and acceptance contribute to enhancing trauma resilience. Therefore, to reduce PTSD symptom severity through the use of mindfulness and acceptancebased smartphone app, it is necessary to first develop mindfulness and acceptance which, in turn, will promote resilience to trauma. Promoting mind fulness and acceptance through the use of an ACTbased smart phone app helps individuals with PTSD to develop nonjudgmental awareness of their traumarelated internal experiences. In turn, this awareness facilitates the processes of resilience to trauma. Examples of such resilience processes include reducing ruminative thinking,51 enhancing emotional processing of traumatic events,52 and using emotion regulation to decrease reactivity to distressing and intrusive posttraumatic triggers.53 Therefore, our findings demonstrate the urgent need to promote mindfulness and acceptancebased inter ventions as resilienceenhancing tools for individuals with PTSD. Importantly, the focus of such interventions on facilitating resilience, rather than on the amelioration of symptoms of mental disorders (such as PTSD), would offer a nonstigmatizing approach and increase access to mental health programs among population groups with high prevalence rates of PTSD aggravated by stigmatized views about mental disorders and traditional mental health services, such as the military veterans.29,54 5 | LIMITATIONS The present study has several limitations. First, the generalizability of the results is limited by sample size, use of convenience sampling, and absence of a control group. However, despite these limitations, our findings provide meaningful evidence on the role of resilience in mindfulness and acceptancebased interventions for individuals with PTSD. In our prospective research aimed at confirming the efficacy of our intervention, we intend to enroll a larger sample and use a randomized controlled trial (RCT) design. The second limitation of the present study is that we did not control for a potential use of other strategies (e.g., other webbased PTSD interventions, mindfulness and nonmindfulness apps, or face toface psychotherapies and groups) that the participants might have complementary employed to manage their PTSD symptoms during the intervention period. All such complementary strategies could have had mediating or moderating effects on the use of the studied app and its outcomes. To rule out this possibility, our prospective RCTstudy will include relevant measures to monitor other strategies

used by the participants during the intervention period and evaluate their effects on our app adherence and outcomes. Third, the generalizability of our findings is compromised by a relatively short (4 weeks) duration of the intervention period and the lack of followup assessments to evaluate the effects of using the app in the postintervention period. Although shortterm mobile app and webbased ACT programs have been reported to cause a significant reduction of psychological distress,55,56 our decision about the 4week duration of our intervention was underpinned by our in tention to initially determine the feasibility and acceptability of the intervention among college student veterans, a population cohort known to have stigmatized beliefs about traditional mental health services.29,57 Yet, the promising results of the present study, such as an increased level of adherence28 and initial significant improve ment of outcomes, support prolonging the intervention period and including longer followup assessments (i.e., at 1 and 3 months postintervention) in our prospective fullypowered RCT study. The fourth and final limitation of the present study is that we did not determine the relationships among specific aspects or factors of mindfulness with resilience, PTSD, and experiential avoidance. This omission is due to the fact that the MAAS, the mindfulness scale we used, has a singlefactor structure.39 In further research, other mindfulness scales with more detailed subscalessuch as, for in stance, the Five Facet Mindfulness Questionnaire58could be used to determine which mindfulness processes (e.g., nonreactivity, ac cepting without judgment, acting with awareness) are the strongest predictors of improvements in resilience, experiential avoidance, and PTSD symptom severity.

6 | CONCLUSIONS As demonstrated by the results of the present study, an ACTbased mobile app is a safe and effective selfhelp tool for college student veterans with PTSD symptoms. Our findings also revealed that, after a4week use of the app, the participants experienced an improve ment in mindfulness, and this improvement was associated with improvements in resilience and PTSD symptom severity. Taken to gether, these results provide preliminary evidence on the mechanism of action of ACTbased interventions on resilience and mindfulness in individuals with PTSD. In further research, it would be necessary to explore specific processes of resilience and mindfulness that im pact PTSD symptom severity.

7 | IMPLICATIONS FOR PSYCHIATRIC NURSING PRACTICE In view of the limitations of the present study discussed above (i.e., small sample size, singlearm study design, and absence of followup assessment), the implications of our findings for psychiatric nursing practice of our findings may also be limited. At the same time, the present study provides some evidence in support of nurses' use of ACT mobile apps as complementary nonpharmacological approaches to enhance resilience in individuals with PTSD. Therefore, along with initiating discussions with their clients about using ACT mobile apps as costeffective and nonstigmatizing options of PTSD management, nurses need to be aware of the preliminary nature of such evidence. This also implies that, in their exploration of complementary non pharmacological options for managing PTSD, nurses may need to be knowledgeable about different levels of evidence (e.g., systematic reviews of random controlled trials vs. one RCT only or one quasi experimental study only) and transparently present this information to their clients. Furthermore, although our findings are preliminary and require further research in a fullscale trial, the use of an ACTbased mobile app intervention may appeal to other population cohorts that, simi larly to college student veterans, hold stigmatized beliefs about mental disorders and conventional mental health services.29 In summary, based on available evidence, our findings allow us to re commend using ACTbased mobile app