Question
How often did enslaved people
escape to the free states?

Explanation:

the railroad is thought to help as many as seventy thousand individuals though estimations vary from 40000 to 100000 Escape From Slavery in the Years between 1800 and 1865 even without the journey was grueling

this null effect is caused by a weak manipulation and ineffective internal validity. in a scientific study, as the one dr. morimoto is trying to perfom, internal validity refers to the confidence placed in the cause-effect relationship and to how well an experiment is done and able to eliminate confounding variables.