reporting subject's height, age, gender and region. Test subjects may have been invited instead of chosen randomly, resulting in sampling bias. Some countries may have significant height gaps between different regions. For instance, one survey shows there is gap between the tallest state and the shortest state in Germany. Under such circumstances, the mean height may not represent the total population unless sample subjects are appropriately taken from all regions with using weighted average of the different regional groups. Different social groups can show different mean height. According to a study in France, executives and professionals are taller, and university students are taller than the national average. As this case shows, data taken from a particular social group may not represent a total population in some countries. A relatively small sample of the population may have been measured, which makes it uncertain whether this sample accurately represents the entire population. The height of persons can vary over a day, due to factors such as a height increase from exercise done directly before measurement (normally inversely correlated), or a height increase since lying down for a significant period (normally inversely correlated). For example, one study revealed a mean decrease of in the heights of 100 children from getting out of bed in the morning to between 4 and 5 p.m. that same day. Such factors may not have been controlled in some of the studies. Men from Bosnia and Herzegovina, the Netherlands, Croatia, Serbia and Montenegro have the tallest average height. Dinka people are sometimes noted for their height. With the Tutsi of Rwanda, they are believed to be the tallest people in Africa. Roberts and Bainbridge reported the average height of in a sample of 52 Dinka Agaar and in 227 Dinka Ruweng measured in 1953–1954. Other studies of comparative historical height data and nutrition place the Dinka as the tallest people in the world. Measurement Crown-rump length is the measurement of the length of human embryos and fetuses from the top of the head (crown) to the bottom of the buttocks (rump). It is typically determined from ultrasound imagery and can be used to estimate gestational age. Until two years old, recumbent length is used to measure infants. Length measures the same dimension as height, but height is measured standing up while the length is measured lying down. In developed nations, the average total body length of a newborn