Using a cell phone while driving can increase the risk of an accident. One reason is that driving while using a cell phone may increase the time it takes to react to many situations. But how much does it affect reaction time? To answer this question, we could estimate the difference in mean reaction times for people who drive while using their cell phones and people who do not.

When we are comparing two population means, we add subscripts to indicate which population (and sample) is being referenced.

In the context of the cell phone reaction time example:

• [latex]\mu_\text{cell phone}[/latex] will represent the mean reaction time of people who use their cell phones while driving.
• [latex]\mu_\text{no cell phone}[/latex] will represent the mean reaction time of people who do not use their cell phones while driving.