A company that sells hair$-$care products wants to determine if a product that combines shampoo and conditioner works better than 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 volunteers will use the shampoo$/$conditioner combination and which ones will use the separate shampoo and conditioner. After using the products for one month, the subjects are asked to rate their satisfaction with the hair products on a scale of $110(1=$ highly dissatisfied and $10=$ highly satisfied$).$What type of sampling is described in this study?