The stopping distance of a certain car traveling at 60 miles per hour follows an approximately normal distribution with a mean of 130 feet and a standard deviation of 5 feet. Approximately what percent of the time does the car stop at a distance of between 120 feet and 140 feet?

The distribution of weights for bags of marbles is shown to follow an approximately normal distribution, as seen in this histogram.

Histogram. The vertical axis is labeled from 0 to .18 by 0.02’s. The horizontal axis is labeled weight of bag, ounces, and has bin widths of 0.2, starting at 8. Beginning at 8 up to, but not including 8.2, height of bar at each interval is 0.005, 0.01, 0.016, 0.026. 0.038, 0.053, 0.069, 0.083, 0.095, 0.101, 0.101, 0.095, 0.083, 0.068, 0.053, 0.038, 0.026, 0.016, 0.009. 0.005. 0. The fifth through sixteenth bars are shaded.

1. What is the approximate mean weight of a marble bag?
2. What is the approximate standard deviation of the weight of a marble bag?

Here is a histogram of a distribution with 50 data points.

1. 80 to 81
2. 83 to 84
3. 88 to 89

Why is it important to randomly assign people to groups in an experimental study?