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Bias in a sampling method is

a. any difference between the sample result and the truth about the population.

b. the difference between the sample result and the truth about the population due to

using chance to select a sample.

c. any difference between the sample result and the truth about the population due to

practical difficulties such as contacting the subjects selected.

d. any difference between the sample result and the truth about the population that

tends to occur in the same direction whenever you use this sampling method.

e. racism or sexism on the part of those who take the sample.

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.

Which of the following best describes the design of this experiment?

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).