 Variables can be classified into categorical or qualitative variable, numerical or quantitative variable. Categorical variables represent groupings of some kind. The numerical or quantitative variable quantifies some amount. Within categories, we have nominal and ordinal, which is classified based on the rank order system in the outcome variables. When there is no rank order, such as gender or smoking, present or absent, then it will be considered as nominal variable. It can be again divided into binomial or polynomial. Ordinal variables follow a rank order in its outcomes. It can be either a like-at scale or mild-moderate severe under numerical variables. If the outcomes are only whole numbers, then it is otherwise called as discrete or impulse. Usually the number of students, if it follows a decimal point, then it becomes a continuous variable. Under continuous variable, we have interval and ratio. In interval scale, there will be equal spacing between the outcomes. In ratio scale, there will be equal spacing between the outcomes and also there will be true zero. Example for interval scale is the temperature in Celsius or Fahrenheit. The example for ratio scale will be weight, volume, distance. Epidemiologically, variables are classified into independent or dependent variables or otherwise called as exposure variables or outcome variables.