Shari is trying to determine how music affects a person’s pulse rate. she has her mother listen to classic rock at a high volume for 30 minutes, her younger brother listen to classical music quietly while he sleeps at night, and her grandmother listen to two hours of rap at a medium volume. she records each person’s pulse before and after listening to the music. what is missing from shari’s scientific investigation?

Shari's scientific investigation is missing controlled variables.

Explanation:

A good scientific investigation should change only the factor, which is examined. In the given case, Shari has no control for volume, the volume of the music may influence the rate at which the heart rate of the listener will elevate. She needs to determine the pulse of the individuals after the same time of listening.  

She has no control for age, as a grandmother would have reacted to loud music distinctly in comparison to a teenager. All her subjects should be of the similar age group, and she even has no control for wakefulness, as an individual would react to music distinctly when he or she is awake in comparison to when he is asleep.  

it's filtration, but this is an assumption so my apologies if i got this