Unknown - Unknown Risks

John, Thanks for your input. I am not aware of the normal weather conditions in that area but is it normal to have hurricanes and flooding there ? Is yes, then this might be something that would have been taken into consideration in Contingency reserves. Can you shed more light on this please ?

In the US, areas in flood plains should expect be flooded about every 10 years, flood hazard areas about every 100 yrs. In the case of flooding, the area was in a flood hazard area so we had a one in 200 chance of flooding on a half year project. That's about 2.5 standard deviations on a normal curve, so perhaps we should have considered it. Snowstorms and hurricanes are also somewhat statistically probable, and certainly seasonal, so we can perhaps lump them and consider them "storms" and assign a storm reserve based on some formula.

Another good example Pankaj, thanks. You could not avoid the delay in schedule at all ?

Unknow-unknow risks often arise. The real challenger is to avoid cognitive ones arising. I mean, you have a problem when sponsor or stakeholders recognize you - as a PM - could be identified and became ready to react early. If an unknow-unknow Risk arises and everyone agrees it was impossible to identify under normal conditions, workaround tasks are well accepted.