violence. Systematic variation in response patterns could also have contributed to the divergence. People experiencing severe outcomes that are seen as socially undesirable—aggressive tendencies or indicators of risk for mental health problems, for example—may have been more likely to have reported their experiences honestly in a self-administered online questionnaire than in an in-person interview. A different set of respondents, who were not themselves experiencing severe outcomes but perhaps had friends or relatives in difficult circumstances, may purposely have reported more severe outcomes than they experienced personally with the goal of influencing resource allocation toward the needs they observed. While the PPCS and Facebook survey were not designed to isolate the effects of these different mechanisms, we explore each of them further in the sections that follow.

Were people who experienced more severe outcomes more likely to complete the Facebook survey?

The PPCS team secured high response and completion rates for the detailed adult individual interview, but the Facebook survey reflected the nonresponse and dropout challenges that are common to online questionnaires. Almost 96 percent of the adults selected for the PPCS individual interview agreed to participate, and virtually all of those adults completed the questionnaire. In contrast, about 12 percent of the people who clicked on the Facebook-advertised link (themselves a fraction of the Facebook users for whom the advertisement appeared) agreed to start the survey, and of those respondents only 54 percent remained to comprise the final sample.

The Facebook and PPCS samples differ substantially in some observed characteristics (further discussed below in the section on weighting strategies), and it is likely that they also differ in important unobserved characteristics given the high nonresponse rates of the Facebook survey. One plausible unobserved selection mechanism is interest in the survey topics. According to the introduction to the Facebook survey that preceded agreement to participate, the questionnaire was “about the psychological conditions of Palestinians living in the West Bank and Gaza” and the research team was “trying to understand how people have been affected by the conflict that took place last May.” Other topics mentioned include background information, employment status, food security, and “exposure and reactions to recent events.” The Facebook survey almost certainly achieved higher response rates among people for whom these topics—particularly the psychological conditions of Palestinians and effects of the May 2021 conflict— were interesting and relevant. This likely included people who had experienced the effects of the May 2021 conflict at close range (including through the experiences of friends and relatives), potentially inducing selection bias in measures of conflict exposure and associated variables. The associations between experiences of conflict exposure, mental health, and food insecurity are nuanced: mental health responses to adverse events vary from person to person, and the specific conflict events investigated in the Facebook survey represent a small subset of the overall conditions of direct and structural violence in the West Bank and Gaza. However, given the observed differences in estimates between the two surveys, we think it possible that the people who chose to respond to the Facebook survey included a higher proportion (relative to the population) of those who experienced greater exposure to the May 2021 escalation as well as more severe outcomes in the other survey topics.

While the introductory letter for the PPCS included a similar description of the core topic of the survey, nonresponse bias related to topic interest should be much smaller in this case because almost all adults 5