Emma is planning an experiment to examine whether reading to children increases their vocabulary size. she plans to measure the vocabulary size of a group of 18-month-old children, read to them three times a week for 3 months, then measure their vocabulary size again. which threat to internal validity should she be most concerned about

   She could add a comparison group

Explanation:  Emma will conduct an experiment with children 18 months of age and read to them three times a week for three months and then measure their vocabulary, although without reading, the vocabulary will change in three months, naturally. Therefore, in order to determine the impact of reading on vocabulary change, Emma should also have a comparable group to which she will not read three times a week for three months, and to compare its vocabulary with the group to which she is reading. Thus, she will see how much reading has influenced the vocabulary change when compared to the natural vocabulary change in the non-reading group. In other words, whether the change in the group to which she was reading is greater, does that group have a larger word pool in its vocabulary than the group to which she did not read.

the earth’s crust is broken into separate pieces called tectonic plates (fig. 7.14). recall that the crust is the solid, rocky, outer shell of the planet. it is composed of two distinctly different types of material: the less-dense continental crust and the more-dense oceanic crust. both types of crust rest atop solid, upper mantle material. the upper mantle, in turn, floats on a denser layer of lower mantle that is much like thick molten tar.

each tectonic plate is free-floating and can move independently. earthquakes and volcanoes are the direct result of the movement of tectonic plates at fault lines. the term fault is used to describe the boundary between tectonic plates. most of the earthquakes and volcanoes around the pacific ocean basin—a pattern known as the “ring of fire”—are due to the movement of tectonic plates in this region. other observable results of short-term plate movement include the gradual widening of the great rift lakes in eastern africa and the rising of the himalayan mountain range. the motion of plates can be described in four general patterns:

collision: when two continental plates are shoved together

subduction: when one plate plunges beneath another (fig. 7.15)

spreading: when two plates are pushed apart (fig. 7.15)

transform faulting: when two plates slide past each other (fig. 7.15)