A company that sells hair-care products wants to estimate the mean difference in satisfaction rating for a product that combines shampoo and conditioner compared with a shampoo and conditioner used separately. A researcher recruits 60 volunteers and pairs them according to age, hair color, and hair type. For each pair, the researcher flips a coin to determine which volunteer will use the shampoo/conditioner combination and which one will use the separate shampoo and conditioner. After using the products for one month, the subjects will be asked to rate their satisfaction with the hair products on a scale of 1 – 10 ( 1 is equal to highly dissatisfied and 10 is equal to highly satisfied). The mean difference in satisfaction ratings open paren Combined minus Separate close paren is calculated. What is the appropriate procedure?