You wonder if TV ads are more effective when they are longer or repeated more often or both. So you design an experiment. You prepare $30-$second and $60-$second ads for a camera. Your subjects all watch the same TV program, but you assign them at random to four groups. One group sees the $30-$second ad once during the program; another sees it three times; the third group sees the $60-$second ad once, and the last group sees the $60-$second ad three times. You ask all subjects how likely they are to buy the camera.

(a) This is a randomized block design, but not a matched pairs design.

(b) This is a matched pairs design.

(c) This is a completely randomized design with one explanatory variable (factor).

(d) This is a completely randomized design with two explanatory variables (factors).

(e) This is a completely randomized design with four explanatory variables (factors).

Longer or more often repeated television commercials are more successful.

When random sampling is not used, bias or systematic flaws in the way the sample represents the population can occur. Voluntary response samples, in which respondents choose their own respondents, and convenience samples, in which people who live nearby are included in the sample, are particularly biassed.

The topic asks if TV commercials are more effective when they are longer, repeated more frequently, or both. As a result, an experiment was carried out. Thus, four treatment groups were included in the experiment, with two explanatory variables: duration ($30$ or $60$ seconds) and repeats ($1$ or $3$). It is a perfectly randomized design because all individuals were randomly assigned to one of the four groups. As a result, option (d), that this is a completely randomized design with two explanatory variables, is correct.