The overall study design of the NMHS was multi-stage, stratified, random cluster sampling technique, with random selection based on Probability Proportion to Size at each stage (MSRS-PPS).

A multi stage sampling was adopted in each state and each selected state of India constituted the sampling frame. The districts and talukas within the states constituted the Primary and Secondary Sampling Units, respectively. Each named inhabited village as per the Census 2011 constituted a rural cluster, while a census enumeration block represented an urban cluster and individuals within the identified households formed the unit of analysis. The numbers of rural, urban metro and urban non-metro clusters were selected based on their respective proportions for the state. Within each cluster all eligible members (>=18 years) within the households selected using systematic random sampling method were interviewed. Thus, in total 34802 adults and about 1191 adolescents drawn from 12 states were interviewed.

The study instruments collected sociodemographic information including completed age, gender, education, occupation, income (house-hold and individual) and marital status. For assessment of mental morbidity, the Mini International Neuro-Psychiatric Inventory (MINI) adult version and the MINI-Kid version were used for adults and, older children and adolescents, respectively. In addition, additional questionnaires for tobacco use (Fagerstrom questionnaire) and to screen for Epilepsy, Intellectual Disability (ID) and Autism Spectrum Disorders (ASD) were also incorporated. Further, questionnaires on health care utilisation, assessment of disability (modified Sheehan’s scale) and socioeconomic impact of illness were used in the study. All data collection instruments were translated into the local languages of each of the surveyed states. Translated MINI instruments which were already available along with the other translated instruments were checked for social and cultural appropriateness, back -translated and then appropriate changes made for the final versions, which were then field-tested before use.

A team of 8 – 10 well trained field data collectors undertook data collection in each state based on micro planning steps after finalising field logistics. Training for the NMHS was conducted at 3 levels; first at NIMHANS for the core team, second for all PIs and Co-PIs from the selected states and third for state data collection teams. The data collection teams were trained for a period of 8 weeks. This involved two weeks of monitored data collection in the field. All the state teams were trained based on a uniform standardised training schema which enabled data collection in a standardized manner in all the states. Approvals were obtained from all local authorities and using the door knock method the field staff administered the questionnaires, using hand held tablet computers (Dell Venue 8 Pro 5000 series).