This process helps assure that the groups are similar to each other when treatment begins.
Therefore, any post-study differences between groups shouldn’t be due to prior differences.
In this case, we’re talking about random Random assignment might involve flipping a coin, drawing names out of a hat, or using random numbers.
All subjects should have the same probability of being assigned to any group.
Scenario 2: We use random assignment so the treatment and control groups start out with roughly equal levels of physical activity.
Activity still affects bone density but it is equally spread across the groups.
Because activity increases bone density, the higher activity in the treatment group may account for the greater bone density compared to the less active control group.
Because it is not in the model, activity is a confounding variable that makes the jumping exercise appear to be significant when it might not be.