Which was an obstacle in building the National (Cumberland) Road? American Indians attacked the crews building the road.
The Spanish blockaded the Mississippi River.
Roads had to be built by hand with picks and shovels.
Workers had to build a bridge over the Grand Canyon.

Using mr smith and his id, identify two potential internal validity threats that might affect the a—b design. tell me how that threat might impact conclusions in the a—b design. note #1: you cannot use maturation (since i already did that one for note #2: there are some mr smith id threats that won’t have much impact in a single case design.