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.
What is a 'Simple Random Sample'. A simple random sample is a subset of a statistical population in which each member of the subset has an equal probability of being chosen. An example of a simple random sample would be the names of 25 employees being chosen out of a hat from a company of 250 employees. In this case, the population is all 250 employees, and the sample is random because each employee has an equal chance of being chosen.