that the share with access to mobile phones remained high following the start of the conflict (although some households reported sharing a mobile phone),18 the geographic coverage of the survey reaches the vast majority of the country,19 and the WFP survey itself demonstrates that the number of mobile phones owned by households has largely not changed at the national and governorate levels.20 Importantly, the shocks on which this analysis focuses all affect regions where the number of mobile phones has not changed since the last nationally-representative estimate (e.g., Abyan, Al Jawf, Shabwah, etc.).21 Another potential challenge to the generalizability of results is sample selection (i.e., nonrandom non-response). For example, in random digit dialing phone surveys in the United States of political preferences, the surveys are generally good at predicting party affiliation and many other political attributes relative to traditional household surveys but over-predict the amount of civic engagement due to differences in who is most likely to respond to a phone survey (e.g., Abraham et al. 2009).

To the degree that we are able to assess this issue in such a data- and evidence-scarce environment, we validate the WFP survey’s representativeness of the broader mobile phoneusing population by demonstrating that the survey is capturing trends that are independently corroborated by other sources. We demonstrate that the mobile phone survey does in fact detect large declines in many welfare outcomes and access to basic services that are consisFurthermore, this figure remained high for the rural population (81 percent), the population living below the poverty line (77 percent), and the population of all governorates (over 60 percent for each). Authors’ calculations using the 2014 Household Budget Survey.

> 18Registration for the  World Bank’ s  cash  transfers  program  being  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   (see   (accessed September 2018): http://www.worldbank.org/en/news/press-release/2017/05/19/new-world- bank-support-to-address-food-

> insecurity-in-yemen-aims-to-reach-9-million-yemenis); 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 (see (accessed September 2018): http://documents.wfp.org/stellent/groups/internal/documents/projects/wfp278006.pdf);     and     evidence from the WFP mobile phone survey itself suggests that particularly struggling households are well represented in the sample with nearly 40 percent of the sample being displaced and approximately 41 percent receiving food assistance.

19There are only 16 districts 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 the March 2019 survey, 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 March 2019 survey; and the districts overwhelmingly came from the governorates of Hadramaut and Al Mahrah, where only 39 and 33 percent of districts in each respective governorate had at least one respondent. Importantly, neither of these governorates is the primary focus of the analysis here.

20For each survey, the 95 percent confidence interval for the average number of mobile phones owned by the household is contained in 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 52 percent of the governorate-month observations. However, in the cases where there was not broad overlap between the WFP survey and preconflict data, much of the lack of overlap is in part due to the changes in the composition of the populations due to large-scale migration of IDPs (e.g., TFPM 2017), and also due to the five years that has passed since the last nationally-representative survey.

21All estimates are qualitatively identical when the analysis only focuses on regions where mobile phone ownership has not changed since the last nationally-representative estimate.

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