This chapter describes the different types of data. Most of the data analysis techniques deal with data of two major types: categorical or numerical. Categorical, or qualitative, data can be of two types, either nominal or ordinal. Nominal data refer to the type of data that can only take certain possible values and the order of the categories is not important. Numerical, or quantitative, can be of two types: discrete or continuous. Continuous data refer to data that can take any value in a numerical scale. The resulting types of data censoring during a survival analysis can include; left censored; right censored and interval censored. Types of data transformation include the logarithms, square roots, inverse, or other functions of the data. Data cleaning refers to the process of identifying and correcting inaccurate, incomplete, or inconsistent data.