Assuming you take 27 people from all over the world at random, and all 27 of them have two legs, you can be fairly certain that most humans have two legs. You might also conclude that about 50% are male and about 60% are from Asia, but as traits get less frequent your error bars get bigger and bigger.
Of course actually picking 27 random people is extremely hard, so actual studies compensate by picking more people.
Random does not mean that it's enough to do a proper group comparison, because individual differences, genetics, lifestyle, etc... are additional factors to take in account for dementia related outcomes.
Of course actually picking 27 random people is extremely hard, so actual studies compensate by picking more people.