24.1 Means of two independent samples

A study (Strayer and Johnston 2001; Agresti and Franklin 2007) examined the reaction times of students while driving.

In one study, two different groups of students were used: one group used a mobile phone, and a different group did not use a mobile phone. The reaction time for each student was measured in a driving simulator.

The study uses two groups with different treatments: one group using a mobile phone while driving, and a different group not using a mobile phone while driving.

The data are not paired; instead, the means of two separate (or independent) samples are being compared. (The data would be paired if each student was measured twice: once using a phone, and once without using a phone.)

Consider the RQ:

For students, what is the difference between the mean reaction time while driving when using a mobile phone and the mean reaction time while driving when not using a mobile phone?

The data are shown below.

Think 24.1 (POCI) What are P, O, C and I in this study?
P: Students (this is defined more specifically in the original study). O: Mean reaction time. C: Between those using and not using a mobile phone while driving. I: Yes; the use of a phone (or not) was decided by the researchers.


Agresti A, Franklin CA. Statistics: The art and science of learning from data. 3rd edition. Pearson Education Limited; 2007;
Strayer DL, Johnston WA. Driven to distraction: Dual-task studies of simulated driving and conversing on a cellular telephone. Psychological Science. 2001;12(6):462–6.