Since you are all keen tea- and coffee-drinkers you decide to test the alleged effect on surface staining.

You decide to alternate your toothpaste every week for the duration of 3 months and see if the tooth-staining is affected. You agree to use your smart phone to take a selfie of your smile at the end of each week.

Your idea is to test if the staining is less by the end of the week when you have used the new toothpaste, by simply comparing your selfie smiles.

Other friends find the idea amusing, and agree therefore to join. You end up with 20 participants. Find out More