recorded instance of conflict; and information on port blockages and non‐payment of government salaries was obtained from reports of humanitarian and news agencies on the ground in Yemen.

The food security data were obtained from a mobile phone survey of approximately 2,400 households conducted each month by the WFP. The survey, conducted via random digit dialing, continues to survey households in a governorate until a sufficient number of respondents complete the survey, where the governorate targets are based on the governorate’s share of the total population. The survey is representative of the mobile phone‐using population and provides governorate‐level estimates of five commonly‐collected food coping strategies used by households with poor access to food‐ restricting consumption, reducing the number of meals, relying on less expensive foods, borrowing to purchase food, and limiting portion sizes. Thus, the data allow for a regionally‐disaggregated analysis of food access each month over nearly the entire course of the conflict.

However, there are two issues to note with these data. First, the survey can only be representative of the mobile phone‐using population following the beginning of the conflict. However, mobile phone ownership was very high prior to the conflict (even among relatively disadvantaged groups),<sup>13</sup> anecdotal evidence suggests that even struggling households have found innovative ways to maintain mobile phone access despite the lack of access to network electricity and the dramatic decline in overall welfare,<sup>14,15</sup> and a comparison of the number of mobile phones owned in the WFP surveys and the population‐level estimates before the conflict are actually very similar (Tandon and Vishwanath 2019).

Second, there could be sample selection (i.e., non‐random non‐response) of who responds to the survey methodology and the averages analyzed here can only speak to the actual sample as opposed to populations in which we might have more interest.<sup>16</sup> Although this is a difficult issue to directly address, there is evidence that the WFP surveys detected many of the significant changes in welfare relative to the pre‐conflict population estimates that large segments of the population have been reported to have experienced during the conflict, and the survey is able to roughly identify the areas that are receiving the largest amount of food assistance as identified by 2018 population‐level estimates by the UN and the humanitarian community (IPC 2018; Tandon and Vishwanath 2019). Thus, there is some corroboration 13 Approximately 85 percent of the population resided in a household that owned a mobile phone and would be represented in the household‐level survey conducted by the WFP. In no governorate was this share lower than 60 percent, and the share was 77 and 81 percent respectively for poor households and rural households.

14 The survey reaches a large share of struggling households, where approximately one‐third of the respondents report being internally displaced and upwards of 40 percent of the population reports receiving food assistance. Additionally, surveys of food aid recipients performed by the WFP and the registration procedure for the World Bank’s cash transfers program being implemented by UNICEF (which covers nearly one‐quarter of the entire population and is aimed at relatively poorer households) both find that very large shares of beneficiaries have access to mobile phones. For example, see (accessed January 2019): <u>http://documents.wfp.org/ stellent/groups/internal/documents/projects/wfp278006.pdf and 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).</u> 15 Authors’ calculations from the unit‐level WFP mobile phone survey conducted in November 2017 demonstrate that the vast majority of the population relies on either solar power or generators for their primary energy source. 16 Approximately 8 percent of phone numbers do not respond to two straight surveys, which results in their removal from the sample. However, the WFP is unable to observe whether a phone number exists or not in the random digit dialing procedure, and thus we are unable to identify the share of active phone numbers that do not respond to the survey.

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