Answer:
Yes, I would recommend those sleeping 9 hours to consider sleeping 6 or 7 hours.
Explanation:
First of all, the sample of the study is very representative because the amount of observations is quite big: 1.1 million!. This means that the results of the study have a very low margin of error, and therefore, can be trusted.
Secondly, the calculations are simple, but correct. The easiest way to determine the mortality rate is to divide the number of people who died by the number of people who are still living, and who belong to the same sample.