a useful indicator of a country's level of health or development, and is a component of the physical quality of life index. However, the method of calculating IMR often varies widely between countries and is based on how they define a live birth and how many premature infants are born in the country. Reporting of infant mortality rates can be inconsistent, and may be understated, depending on a nation's live birth criterion, vital registration system, and reporting practices. The reported IMR provides one statistic which reflects the standard of living in each nation. Changes in the infant mortality rate reflect social and technical capacities of a nation's population. The World Health Organization (WHO) defines a live birth as any infant born demonstrating independent signs of life, including breathing, heartbeat, umbilical cord pulsation or definite movement of voluntary muscles. This definition is used in Austria, for example. The WHO definition is also used in Germany, but with one slight modification: muscle movement is not considered to be a sign of life. Many countries, however, including certain European states (e.g. France) and Japan, only count as live births cases where an infant breathes at birth, which makes their reported IMR numbers somewhat lower and increases their rates of perinatal mortality. In the Czech Republic and Bulgaria, for instance, requirements for live birth are even higher. Although many countries have vital registration systems and certain reporting practices, there are many inaccuracies, particularly in undeveloped nations, in the statistics of the number of infants dying. Studies have shown that comparing three information sources (official registries, household surveys, and popular reporters) that the "popular death reporters" are the most accurate. Popular death reporters include midwives, gravediggers, coffin builders, priests, and others—essentially people who knew the most about the child's death. In developing nations, access to vital registries, and other government-run systems which record births and deaths, is difficult for poor families for several reasons. These struggles force stress on families, and make them take drastic measures in unofficial death ceremonies for their deceased infants. As a result, government statistics will inaccurately reflect a nation's infant mortality rate. Popular death reporters have first-hand information, and provided this information can be collected and collated, can provide reliable data which provide a nation with accurate death counts and meaningful causes of deaths that can be measured/studied. UNICEF uses a statistical methodology to account for reporting differences among countries: