What is the source of data for international indices/reports?

I believe most (if not all) reports & indices prepared by UN agencies, WEF, Welthungerhilfe,etc. (eg. HDI,MPI,GHI,etc.)are based on survey data provided by various national governments. I would like to know if there are any indices prepared on own independent surveys (examples required) & whether the sample size of those surveys is large enough. Do these international institutions have so much global staff to do the surveys at local levels?