These magma plumes rise to the surface and form chains of volcanoes. Composite volcano magma contains more silica, SiO 2, than that of a shield volcano and is therefore stickier (more viscous). This stickiness plugs up the volcano, causing pressure to build-up. The result is an explosive, dangerous eruption.
It results in a biased sample, a non-random sample of a population (or non-human factors) in which all individuals, or instances, were not equally likely to have been selected. If this is not accounted for, results can be erroneously attributed to the phenomenon under study rather than to the method of sampling.
1 This may be an extreme form of biased sampling, because certain members of the population are totally excluded from the sample (that is, they have zero probability of being selected). 2  Self-selection bias, which is possible whenever the group of people being studied has any form of control over whether to participate.