The smooth 50th percentile male and female growth curves illustrated above are aggregate values from thousands of individuals sampled at ages from birth to age 20. In reality, a single individuals growth curve shows large upward and downward spikes, partly due to actual differences in growth velocity, and partly due to small measurement errors. For example, a typical measurement error of plus or minus 0.5 cm (0.20 in) may completely nullify 0.5  cm of actual growth resulting in either a "negative" 0.5  cm growth (due to overestimation in the previous visit combined with underestimation in the latter), up to a 1.5 cm (0.6 in) growth (the first visit underestimating and the second visit overestimating) in the same elapsed period between measurements. Note there is a discontinuity in the growth curves at age 2, which reflects the difference in recumbent length (with the child on his or her back), used in measuring infants and toddlers, and standing height typically measured from age 2 onwards. Height, like other phenotypic traits, is determined by a combination of genetics and environmental factors. A childs height based on parental heights is subject to regression toward the mean, therefore extremely tall or short parents will likely have correspondingly taller or shorter offspring, but their offspring will also likely be closer to average height than the parents themselves. Genetic potential and several hormones, minus illness, is a basic determinant for height. Other factors include the genetic response to external factors such as diet, exercise, environment, and life circumstances.
Depending on sex, genetic and environmental factors, shrinkage of stature may begin in middle age in some individuals but tends to be universal in the extremely aged. This decrease in height is due to such factors as decreased height of inter-vertebral discs because of desiccation, atrophy of soft tissues, and postural changes secondary to degenerative disease. Working on data of Indonesia, the study by Baten, Stegl and van der Eng suggests a positive relationship of economic development and average height. In Indonesia, human height has decreased coincidentally with natural or political shocks. Average height around the world
As with any statistical data, the accuracy of such data may be questionable for various reasons:

Some studies may allow subjects to self-report values. Generally speaking, self-reported height tends to be taller than its measured height, although the overestimation of height depends on the reporting subjects 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 10.8 cm (4+1⁄2 in) 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.