Final answer:
The mean of mental illness in the ex-poor group in the "Casino..." natural experiment cannot be determined without specific pre- and post-data. Direct data on mental illness rates following the casino's opening is necessary to evaluate changes.
Step-by-step explanation:
The mean of mental illness in the subjects in the ex-poor group in the "Casino..." natural experiment cannot be determined from the information provided. In such experiments, it isn't enough to know the context; specific data pre- and post-intervention are needed to determine the impact on variables, such as mental illness rates. Factors that could influence mental health outcomes include subsequent economic stability, access to healthcare, and shifts in community dynamics.