1. In a prospective study of depression and dementia, you recruit participants who are between ages 65 and 75 and not suffering dementia. At intake, you give them a screening test for depression. Five years later, you give them the Mini Mental State Examination, which assesses cognitive function. There is no loss to follow-up. Your observed risk ratio is 2.40: elderly people who are depressed have 2.40 times the risk of dementia compared with elderly people who are not depressed over 5 years.

1. If, instead of perfect follow-up, you ended up with 20% fewer participants after 5 years. It turns out that depressed people were more likely to drop out than non-depressed. Explain what effect it would have on your risk ratio?

2. Alternatively, what would be the effect on the risk ratio if the 20% of participants lost to follow-up were all people who were depressed and demented? Explain what effect it would have on your risk ratio?