ported sharing a mobile phone)<sup>21</sup> ; the geographic coverage of the survey reaches the vast majority of the country<sup>22</sup> ; and the WFP survey itself demonstrates that the number of mobile phones owned by households has not changed much at the national and governorate levels.<sup>23</sup> Fourth, it is important to investigate the degree to which sample selection might limit the generalizability of the results. To the degree that we are able to assess in such a dataand evidence-scarce environment, we validate the WFP survey’s representativeness of the broader mobile phone-using population by demonstrating that the survey is capturing trends that are independently corroborated by other sources. The timing of displacement is identical to estimates by the Task Force for Population Movement, where there was an initial surge of displacement in March of 2015 followed by a relatively constant rate of displacement as the location and intensity of violence changed across the country (e.g., TFPM 2019; OCHA 2018; etc.).<sup>24</sup> Additionally, we demonstrate that the mobile phone survey does in fact detect large declines in many welfare outcomes and access to basic

> 21Registration for the World Bank’s cash transfers program (implemented by UNICEF), which covers approximately one-quarter of the total population and is aimed at relatively poorer households, demonstrates the vast majority of households can be reached via phone (e.g., World Bank 2018c). Evidence from different WFP surveys of food aid beneficiaries suggests that the share of food aid recipients that can be reached via mobile phone has remained very high (e.g., WFP 2018b). And evidence from the WFP mobile phone survey itself suggests that particularly struggling households are well-represented in the sample with approximately one-third of the sample being displaced and over one-third receiving food assistance.

> 22See Appendix 6 for a map presenting the number of respondents by district over the entire course of the WFP survey, where there are only two districts out of 333 that do not have any respondents in any of the surveys. Alternatively, the vast majority of districts are represented in any single month of the survey. For example, in November 2017, the survey reached respondents in 264 of the 333 districts in the country- nearly 80 percent of the total. It is important to note that the districts for which there are zero respondents tend to be less populated and in the east of the country. Only 9.2 percent of the country’s 2017 population resided in the 69 districts for which there are zero respondents in the November 2017 survey; and the districts overwhelmingly came from the governorates of Hadramaut and Al Mahrah, where only 39 and 33 percent of districts, respectively, had at least one respondent. Importantly, these governorates are less affected by the conflict and have less displacement than the rest of the country (TFPM 2016) and are not the primary focus of the analysis.

> 23See Appendix 7. The WFP survey began recording the number of working mobile phones owned by each household in March 2016. For each survey, the 95 percent confidence interval for the average number of mobile phones owned by the household is presented and compared to the 95 percent confidence interval prior to the conflict (based on data from the 2014 Household Budget Survey). Despite the large shock to household incomes and energy sources needed to charge mobile phones, the confidence intervals overlap in approximately 78 percent of the governorate-survey observations. In most instances where the two do not overlap, cell phone ownership is lower in the WFP surveys than in the pre-conflict estimate, which is expected given the negative shocks that have occurred since then.

> 24See Appendix 8 for estimates of the timing of the displacement crisis using the mVAM. Specifically, the figure reports the total number of displaced households by month-year of their first displacement, divided by the total number of households interviewed in the survey (18,078).

14