Some people have white coat hypertension. That is, they grow anxious when a person with a white coat and a stethoscope walks into the examining room to take their blood pressure. As a result, their blood pressure increases. A family practitioner believes this is quite common. To test her theory, she puts together a random sample of 50 patients and takes two blood pressure measurements, one when each patient first walks into the room and a second, unexpected, one after about 15 minutes. What statistical test should she use to see if the two blood pressures differ?