A study was designed to evaluate whether socioeconomic factors had an effect on verbalization skills of young children. Four socioeconomic classes were defined, and 20 children under the age of six were selected for the study. The research hypothesis was that the mean verbalization skills would be different for the four classes. The researchers determined that for young children there may be significant gains in verbalization skills over only a few months. Thus, they decided to record the exact age ( in months) of each child. The verbalization skills ( measured by testing) were determined for each child. The data are given here.

a. Plot the sample data. Do verbalization skill and age appear to be linearly related for each of the four groups?
b. Write a first- order model relating verbalization skill to age with a separate line for each socioeconomic group.

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