This is the first study to model the prevalence of selected mental disorders following country-level conflict. In doing so, we draw systematically upon the existing epidemiological research undertaken amongst displaced and conflict-affected populations, published over the past three decades. We have presented the estimated prevalence of depression and PTSD according to levels of population-level political terror, trauma exposure and recency of conflict as they are estimated to have affected six population groups during 2011–2012 Libyan conflict. The statistical modelling reported herein suggest a substantial mental health burden associated with political terror and exposure to traumatic events, reflected in the number of cases of PTSD and depression for exposed populations.
In line with global epidemiological data, depression prevalence is consistently higher than PTSD prevalence across all models. In terms of time effects, our findings demonstrate an overall trend for PTSD prevalence to drop markedly with time since the end of a conflict whilst a reduction in depression prevalence is much more reliant on a reduction in trauma exposure.
Population prevalence estimates of PTSD and depression derived for six population groups identified from areas affected by periods of intense conflict or mass displacement were in the 30% to 40% range within the post-conflict period. These estimates accord with findings from parallel fields of research. A recent systematic review examining major depressive disorder following terrorist attacks suggested that the risk ranges between 20 and 30% in directly affected victims. 
Another review examining PTSD prevalence demonstrated a prevalence of 20–29% following human-made disasters. 
Based on these findings, the estimated prevalence of total cases of PTSD and depression are broadly consistent with the other complex emergency settings.
For the purposes of service planning, we further refined these prevalence estimates by focusing only on those cases that were most likely to be associated with severe levels of impairment by integrating the severity proportions of affective and anxiety disorders recorded by the World Mental Health Surveys. Using this information, severe cases of PTSD were estimated at 9.7%–12.4% and severe depression at 18.3%–19.8% during the immediate post-conflict period. Interpolating these estimates to the 1.24 million Libyans within six population groups identified as being affected by exposure to war trauma and conflict indicates that 123,400 people are likely to have severe PTSD and 228,100 severe depression. Not surprisingly, based upon current WHO estimates, 
the capacity to address mental health needs of the Libyan population falls exceedingly short of what is likely to be required.
The results of the systematic review and meta-regression that form the basis of the current projections indicate that it is not mass conflict in general but rather the population level of exposure to torture, potentially traumatic events and political terror that are the substantive determinants of PTSD and depression prevalence. For this reason, we have limited the attempt to calculate conflict-related mental health projections to those population groups and geographic regions for which there is some information about the extent of trauma exposure.
For the same reasons, we have not attempted to model the conflict related rates for PTSD and depression for the whole of Libya, as the current body of psychiatric epidemiological research has primarily involved small to medium sized population studies of directly affected populations. It is not clear to what extent these estimates can be scaled up to provide prevalence estimates at a national level.
Implications for Mental Health Services
There is increasing recognition of the importance of responding in a timely manner to population-level mental health problems following conflict. 
The mhGAP program and other WHO initiatives 
identify a combination of medical and social interventions delivered principally within the primary care sector as the model most capable of being scaled up to meet population level need. It is important to note that the proposed scale-up of services is for over a 10 year period and not an immediate response goal. 
The ability for resource poor and disrupted health systems to meet population needs is invariably difficult and we have elected to only model severe cases of depression and PTSD. It is recognized this may represent an underestimation of overall need, particularly with regard to psychosocial services for less severe cases. This, combined with the fact that the service requirement model used was developed in countries with much lower prevalence rates than exhibited in our estimates for post-conflict Libya, means our predictions may be conservative; and the actual number of persons requiring service utilisation may be higher.
It is typical for a health system to deteriorate during times of war, with infrastructure becoming degraded and health staff fleeing areas most in need of health services. This would result in even fewer available mental health resources than the WHO pre-conflict estimate for Libya. We have not considered the entire spectrum of mental disorders in our estimations, but rather focused on disorders known to be largely affected by conflict. A comprehensive mental health service would have additional requirements to accommodate the full range of mental disorders.
The importance of Libya returning to a state of peace and stability and a positive post-conflict recovery trajectory for the mental health of the population cannot be overstated. Reducing political terror and trauma is crucial in stabilising prevalence of PTSD and depression. 
Related to this is the documented evidence that many trauma and adversity induced mental disorders, abate once the immediate threat has resolved with a substantial number of people drawing on their natural resilience. 
In that regard, it is apparent from our models that time since the end of the conflict has a significant role in the normalising of PTSD estimates but plays a lesser role for depression estimates. This has important implications for program planning.
It was not the aim of this paper to explore specific forms of treatment interventions or program design, however, the mhGAP action programme 
provides clear recommendations for scaling up care for mental, neurological and substance use disorders in low- and middle-income countries and would be appropriate for mental health programming in the Libyan context. We feel it is important to note that investment in new mental hospitals is not recommended and investments in infrastructure (e.g. acute beds in general hospital) should be paired with commensurate investments in human resources both at the secondary and primary health care level. 
The post-conflict period will see a heavy reliance on international aid and require a well-coordinated response through tools such as 4W mapping. 
The first 4W mapping exercise undertaken in December suggests an over-representation of specialised services provided in the initial response and indicates an increase in non-specialised services and community/family support could be more inline with ISAC guidelines. 
Limitations and Future Research Recommendations
The modelling in this study drew on an existing, heterogeneous body of epidemiological research from conflict-affected countries around the world. One limitation is the challenge of validation of instruments for use in post-conflict environments. The variability in prevalence rates found in psychiatric epidemiology following complex emergencies is partially attributable to differences in context, methodology, and exposure to risk factors which have been identified in previously published work. 
Whilst attempts have been made to account for this variability in our analysis there still remains a significant level of unexplained variability. We have represented this by indicating the wide range of uncertainty around the reported prevalence estimates. Nevertheless, the existing body of post-conflict mental health research provides consistent evidence, as indicated by the meta-analytic findings, of a PTE exposure dose-response association with PTSD and depression that offers useful guidance in projecting needs and possible response models.
It is also important to highlight that measurement errors within the PTS have been demonstrated revealing estimates to be conservatively biased by an absolute order of roughly two. 
This is a reflection of both the scarcity of information and inherent difficulties in measuring human suffering in quantitative terms.
The modelling process was not able to include torture as a variable despite it’s known significance as a risk factor for mental disorders in post-conflict settings 
. While widespread torture has been documented in Libya in two recent Amnesty International reports 
these reports did not provide sufficient detail to estimate the prevalence of this form of abuse within the identified populations. Instead the current analysis applied estimates of country level PTS on the basis of the documented reports of widespread conflict and human rights. We further identified high and intermediate conflict-affected regions whose populations were modelled as being exposed to an intermediate and high level of PTE exposure. These should be seen as estimates only as it is not possible to ascertain the precise level of exposure to PTE’s without detailed epidemiological work amongst the identified populations. Modifications to the populations selected and key parameters chosen (trauma exposure and political terror levels in particular) might be useful if the picture changes as more detailed information becomes available.
It has been necessary to apply surrogate measures in the absence of data for a number of other key areas; ‘affective disorders’ has been used as a proxy for depression in establishing comorbidity rates and ‘anxiety disorder’ severity proportions have been used for PTSD. A lack of statistical power and limitations in carrying out subgroup analysis in the meta-regression modelling necessarily means differences between countries or regions is forfeited and the influence of other environmental 
and specific cultural factors on prevalence estimates is lost. For example, community-specific recovery processes such as social and family support and/or religion could mitigate the effects of trauma.
Population data for affected Libyan regions is also difficult to ascertain in some cases. Without a public domain for verifying official Libyan census data we relied on various web sources to derive best estimates of numbers of people living in areas exposed to the highest levels of violence and trauma. There is no a priori reason to expect that we have either under- or overestimated population sizes, but it is a source of uncertainty.
The need for epidemiological estimates of mental disorders in conflict and post-conflict regions is essential for more effective program prioritisation and planning. Research is needed to fill knowledge gaps and enhance what we have presented in this paper. High quality epidemiological studies from developing and conflict/post-conflict countries are required in order to obtain more accurate baseline prevalence estimates, more statistical power in modelling prevalence estimates, and a better understanding of how cultural and environmental aspects affect modelling. The potential for applying these models, adapted as necessary, to forecasting prevalence estimates in other conflict-affected populations is ready for exploration. It is hoped that with further research and refining of methodologies the modelling will provide even more useful and accurate projections.