10.35 The dispersion, or variance, of haul times on a construction project is of great importance to the project supervisor, because highly variable haul times cause problems in scheduling jobs.

The supervisor of the truck crews states that the range of haul times should not exceed 40 minutes (The range is the difference between the longest and shortest times.) Assuming these haul times to be approximately normally distributed, the project supervisor takes the statement on the range to mean that the standard deviation ( should be approximately 10 minutes. Fifteen haul times are actually measured and show a mean of 142 minutes and a standard deviation of 12 minutes. Can the claim of ( be refuted at the 5% significance level?