AutoTrader.com would like to test if a difference exists in the age of three different types of vehicles currently on the road: trucks, cars, and vans. The following data represent the age of a random sample of trucks, cars, and vans. 
The grand mean for these observations is ________.
Definitions:
Reversal of Treatment
A procedure in single-subject research where an intervention is removed to study its effects on the target behavior or condition.
Single-case Design
A research design that focuses on the response of a single subject to document the effects of a specific condition or intervention.
Reversal Designs
Experimental designs in which an intervention is introduced, removed, and then reintroduced to gauge its effects.
ABAB Design
A type of experimental design in behavioral science that involves alternating between a baseline period (A) and an intervention period (B).
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