For example, one study revealed a mean decrease of 1.54 centimetres (0.61 in) in the heights of 100 children from getting out of bed in the morning to between 4 and 5 p.m. inversely correlated), or a height increase since lying down for a significant period of time (i.e.
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. For instance, one survey shows there is 10.8 cm ( 4 + 1⁄ 2 in) gap between the tallest state and the shortest state in Germany. Some countries may have significant height gaps between different regions.Test subjects may have been invited instead of chosen at random, resulting in sampling bias.Generally speaking, self-reported height tends to be taller than measured height, although the overestimation of height depends on the reporting subject's height, age, gender and region.
Some studies may allow subjects to self-report values.As with any statistical data, the accuracy of such data may be questionable for various reasons: