Confidence interval question

#1
Fr a sample of 100 hypertensive patients, we find that a 95% confidence interval of systolic BP to be [130, 150]. What statement is FALSE?

A- if a drug is given that makes the patients BP more variable, then the resulting CI of the post-treatment BP will be wider.

B-A 90% CI of the BP based on the same data will be narrower.

C-If a drug is given to exactly reduce the BP by 10 units in every patient, then the CI post Treatment will be narrower.

D-A 95% CI on a random subset of 60 patients is going to be wider.


I'm having trouble wrapping my head around this problem. I thought the answer was C, but now I'm questioning myself. Any thoughtS??
 

Karabiner

TS Contributor
#2
C tells you that the CI will be narrower if the mean blood pressure decreases by 10. You should perhaps check the formulas for the calculation of standard errors and CIs - is it the sample mean on which the width of the CI depends, or is it sample variance, or maybe both?

With kind regards

Karabiner