Shampoo Composition
Choosing a good shampoo is easy once you know what to look for. The FDA regulates all food and drug products ingredient lists. Their primary stipulation is that ingredients be listed by quantity from largest to smallest.
Generally, shampoos are 80-90% water and contain 2-8% detergents and foaming agents and about 1% fragrance and preservatives. Based on these numbers alone it's not easy to choose a good shampoo from a bad one. In the end, it often comes down to personal preferences such as fragrance and foaming ability.
Anionic surfactants are predominately used in shampoos and other personal care products. These types of surfactants are relatively inexpensive but offer excellent cleansing properties and will rinse easily from your hair.
Unfortunately, some can be harsh and overtime may irritate the scalp. This irritation can take many forms but more commonly leads to dry, flaky scalp. To combat this, most manufacturers add other surfactants and ingredients to reduce skin irritation. You can learn more about the active ingredients here.