The article “Effects of Too Much TV Can Be Undone” (USA Today, October 1, 2007) included the following paragraph:

Researchers at Johns Hopkins Bloomberg School of Public Health report that it’s not only how many hours children spend in front of the TV, but at what age they watch that matters. They analyzed data from a national survey in which parents of 2707 children were interviewed first when the children were 30–33 months old and again when they were 51 2, about their TV viewing and their behavior.

a. Is the study described an observational study or an experiment?

b. The article says that data from a sample of 2707 parents were used in the study. What other information about the sample would you want in order to evaluate the study?

c. The actual paper referred to by the USA Today article was “Children’s Television Exposure and Behavioral and Social Outcomes at 5.5 years: Does Timing of Exposure Matter?” (Pediatrics [2007]: 762–769).

The paper describes the sample as follows:

The study sample included 2707 children whose mothers completed telephone interviews at both 30 to 33 months and 5.5 years and reported television exposure at both time points.

Of those completing both interviewers, 41 children (1%) were excluded because of missing data on television exposure at one or both time points. Compared with those enrolled in the HS clinical trial, parents in the study sample were disproportionately older, white, more educate, and married.

The “HS clinical trial” referred to in the excerpt from the paper was a nationally representative sample used in the Healthy Steps for Young Children national evaluation. Based on the above description of the study sample, do you think that it is reasonable to regard the sample as representative of parents of all children at age