Tracy and Eric are timing how fast marbles roll down an inclined plane . They plan to do this 5 times , and then take the average time for their results . What would be the BEST way to improve the accuracy of their data?

The accuracy can be improved by making sure no external force is applied, i.e the marbles should accelerate under gravity only during each trial.

Explanation:

Accuracy is a term used to describe the relationship between a measured value to the real value. When a measured value is accurate, it implies that there is little or no error in the value; thus it is equal to the real value or very close to it.

Since this experiment is to determine the speed with which the marble roll down the plane, they should ensure uniform acceleration through out the  trials. The marbles should be allowed to move under the influence of gravity only without applying any external force.