What are the limitations of this study?-

I am tackling the study of the company, where I know that:

A financial firm is interested in the impact that caffeine has on an employee’s sleeping patterns.

• Participants are allocated to either Drink A (regular coffee) or Drink B (decaffeinated coffee),
• The participants do not know which Drink is which
• They must only drink the coffee they have beenallocated during the period of the study.
• Participants will record their number of hours of sleep per night at the beginning and end of the study.

The variability in number of hours of sleep is likely to be similar to previous studies, and the estimated standard deviation is 3.5 hours of sleep. If the difference between the hours of sleep was at least 1.5 hours on average, this would represent a clinically important difference between the drinks.

I would like to know what are the limitations of this study and what do they cause? So far I came up with the unknown variability (as we dont know the previous study) and the fact that A p value, or statistical significance, does not measure the size of an effect or the importance of a result. Is there something else? Thank you!

The number of hours of sleep is a factor of multiple independent variables including person, time , month, age etc. You want to know the effect of one of these variables.