After researching the possible effects of music, Elaina proposes that if people listen to faster-paced music, their pulse rates will increase more than if they listen to slower music. Her mother listens to classic rock at a high volume for 30 minutes, her younger brother listens to classical music quietly while he sleeps at night, and her grandmother listens to two hours of rap at a medium volume. Elaina records each person's pulse before and after listening to the music.

What is missing from Elaina's scientific investigation?

A testable hypothesis naming independent and dependent variables

An experiment that takes initial and final measurements

Controlled variables with only one varied independent variable

Experimental data that can be recorded and analyzed

The scientific investigation performed by Elaina is missing control variables.

An ideal scientific investigation should vary only the factor, which is examined, like the volume of the music, while all the other factors should be kept constant.

She hasn't controlled for volume, the volume of the music may influence the rate at which the heart rate of the listener will elevate, in the case, and she should have determined the pulse rate of each individual after the similar rate of listening.

She hasn't controlled for age, as an older person would have reacted to loud music differently from a teenager, that is, all her subjects should be of about the similar age group.

She hasn't controlled for wakefulness, that is, each individual would react to music distinctly when asleep and when is awake.