A study comparing smokers to non-smokers history as a risk factor for Parkinsons Disease reports an incidence rate ratio of 0.3 comparing smokers to non-smokers.

1. What is the study design?

2. If the association is real, do these findings indicate an increase or decrease in risk for Parkinsons disease?

3. A case-control study comparing Alzheimers cases to controls reported cases were less likely to have reported past smoking. Then, what kind of bias may explain these results?