They are conventionally measured on a mass percent basis. Of the two, alpha acids are the most dominant in bitterness contribution. Both alpha and beta acids are not soluble in water, in order to allow them to contribute to the bitterness of the beer, they must be modified. This is accomplished with alpha acids by a sustained boil. Hops are broadly classified into two groups.
Noble hops are low in bitterness and have a pleasant aroma and flavor.
High-alpha hops have high bitterness but a less pleasant aroma. The high-alpha hops can be used more economically due to the less amount required. However, the flavor of the beer is slightly sacrificed. Years ago, the recipes for beer would call for hops measured in ounces. Since the bitterness for each type of hop was not accounted, the beer's flavor was inconsistent from batch to batch.