The ability of households and individuals to access food (one of the key aspects of ' food security') is an important welfare dimension that poses important challenges for objective measurement. This paper describes the Rasch model-based procedures developed to define the eight-item Food Insecurity Experience Scale (FIES) as a contribution towards the establishment of an indicator for global monitoring of food insecurity. Experiential food insecurity survey data, collected by FAO from nationally representative samples of the adult population, once every year in 2014, 2015 and 2016 from 153 countries or territories, are used to develop methods to estimate cross-country comparable prevalence rates of moderate and severe food insecurity. A Rasch model-based scale was estimated separately for each country and data were assessed for consistency with model assumptions. To ensure cross-country comparability, a procedure based on the median normalized severities of each of the eight FIES items was used to define a global reference scale, against which measures obtained in each country can be separately calibrated. Calibration is obtained by equating the mean and standard deviation of the severity parameters of the items that appear to be common between the national and the reference scale, and thus used as anchoring points for the metric. Data showed sufficient consistency with the Rasch model assumptions to produce reliable measures of the prevalence of food insecurity in each country. Calibration was possible using 4 or more items as anchoring points in 151 of 153 (98.7%) of the cases, and 6 or more items in the vast majority of them (121 cases). Concurrent validation of the estimates of prevalence of food insecurity at national level was obtained by comparing the FIES-based indicator with other established indicators of social (under) development. National prevalence rates of moderate-or-severe food insecurity obtained by FAO correlate well with the prevalence of undernourishment and with several widely used indicators of national income, health, and well-being. The proposed calibration method can be applied to other existing experience-based food security scales that use similar items, thus affording the possibility to use data collected with those instruments to produce internationally comparable measures of the prevalence of food insecurity. Pending broader adoption of the FIES or compatible experience-based food security scales worldwide, countries could choose to use the 2014-2016 results obtained using the data collected by FAO as the baseline to monitor progress towards Target 2.1 of the recently established 2030 Agenda for Sustainable Development.
