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The given statement is:

A driver's ability to detect highway signs is an important consideration in highway safety. In his dissertation,High way Construction Safety and the Aging Driver, S. Younes investigated the distance at which drivers can first detect highway caution signs. This distance is called thedetection distance. Younes analyzed the effect that sign size and sign material have on detection distance. Drivers were randomly assigned to one combination of sign size ( small, medium, and large ), and sign material ( 1, 2, and 3 ). Each driver covered the same stretch of highway at a constant speed during the same time of day, and the detection distance, in feet, was determined for the driver's assigned caution sign.

Taking into consideration the given designed experiment, Experimental units are the drivers that are being studied in this study.

Here, the factors include: sign size and sign material.

Because researchers analyzed the effect that sign size and sign material have on detection distance.

The three levels of sign size include:

1. small
2. Medium
3. Large

The three levels of sign material include:

1, 2 and 3.