According to a treaty with the United States, Canada may withdraw one-third of the water from the Niagara River before it reaches Niagara Falls. Canada uses a canal and two tunnels to take water to the Sir Adam Beck hydroelectric generating stations downstream. The geology of the area is complex, including a buried gorge and flaky shale that crumbles and requires the roof of the tunnel to be reinforced as it is dug. Before work on the second tunnel started, the geologists conducted many surveys and discussed the results with the engineers. Suppose that, when they finally came up with an estimate of the cost of the tunnel, there was a 20% probability that the cost would be 30% higher. What type of probability assessment did they use?