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

Answer: 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.