I've been reading a news report on the results of a study in which previous stress experience was shown to increase the risk of both PTSD and major depression after a disaster (earthquake). It's hard for me to understand two points that seem to contradict each other:
Further analyses showed that prior disaster exposure was not a significant predictor of postdisaster PTSD. Nevertheless, for every unit increase in prior nondisaster stressors, the odds of developing postdisaster PTSD increased (odds ratio [OR], 1.21; 95% confidence interval [CI], 1.08 – 1.37; P = .001).
I take this to mean that for every unit measured using the List of Threatening Experiences, a questionnaire with a maximum score of 12, the risk of PTSD increases. If a person has a score of 1 his risk of PTSD is higher than if he has a score of 0, and so on, with the highest risk reached at score 12.
But further on the text says that:
As such, individuals who have experienced several stressors over the course of a lifetime are at higher risk of developing a psychiatric disorder. This was the case with PTSD, in which exposure to at least four previous manageable stressors was associated with greater odds of developing postdisaster PTSD. For MDD, on the other hand, there was a distinct dose-response relationship between the number of manageable predisaster stressors and the risk for postdisaster MDD.
These two statements seem to contradict each other. Either the risk of PTSD increases for each unit of the LTE, or it increases only for persons who have more than 4 stressfull experiences in their life history.
Here's the original publication (behind a paywall): Assessing the relationship between psychosocial stressors and psychiatric resilience among Chilean disaster survivors (Fernandez et al., 2020)
Here's one figure from the publication: