http://www.paho.org/English/HDP/HDD/nhahenderson.pdf

"... In most developing countries household health spending accounts for the largest single source of financing entering the health sector. Accurate estimation of this critical part of the sector's funding base is what distinguishes National Health Accounts (NHA) from other more conventional health expenditure studies, and is the one that presents, typically, the greatest problems in estimation.As with any other component of the NHA, estimates of household spending are best developed using information from both the demand and supply sides. For example, from data on household consumption and from data on revenues received by providers. In practice, the primary source of data in most countries will consist of household surveys. However, wherever possible this needs to be supplemented by data form the supply side. Although household surveys have long-been used to estimate private health expenditure, NHA studies have consistently shown that household survey estimates require adjustment and caution in use. In doing so, there is considerable experience in evaluating the quality of household survey estimates and in using various approaches to adjusting these estimates...."" (The Authors)