Processing categorical data
Strings usually represent categorical data in tabular data. Each unique value in a categorical feature represents a quality that refers to the example we are examining (hence, we consider this information to be qualitative whereas numerical information is quantitative). In statistical terms, each unique value is called a level and the categorical feature is called a factor. Sometimes you can find numeric codes used as categorical (identifiers), when the qualitative information has been previously encoded into numbers, but the way to deal with them doesn't change: the information is in numeric values but it should be treated as categorical.
Since you don't know how each unique value in a categorical feature is related to every other value present in the feature (if you jump ahead and group values together or order them you are basically expressing a hypothesis you have about the data), you can treat each of them as a value in itself. Hence...
