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.
Bias -- a loss of balance and accuracy in the use of research methods. It can appear in research via the sampling frame, random sampling, or non-response. It can also occur at other stages in research, such as while interviewing, in the design of questions, or in the way data are analyzed and presented.