 Statistically, variables can be classified into two types. 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 in the outcome variables such as gender or smoking, present or absent, then it will be considered as nominal variable. Based on the outcomes, it can be again divided into binomial, when it has two outcomes, or polynomial, more than two outcomes. Ordinal variables follow a rank order in its outcomes. It can be either a Likert scale or it can be mild, moderate, severe in clinical classifications. Under numerical variables, if the outcomes are only whole numbers, then it is otherwise called as discrete or integer variables, where we have counts, usually the number of students, number of family members, number of bacteria in the colony. If it follows a decimal point, then it becomes a continuous variable. Under continuous variable, we have two types, interval and ratio. In interval scale, there will be equal spacing between the outcomes, but there may not be true zero. In ratio scale, there will be equal spacing between the outcomes, and also there will be true zero. An example for interval scale is the temperature in Celsius or Fahrenheit and IQ test values. Here, there will be equal spacing, but there may not be absolute zero. The example for ratio scale will be weight, volume, distance. This classification of variable is more of a statistical classification. Epidemiologically, variables are classified into independent or dependent variables, or otherwise called as exposure variables or outcome variables. And in between, we have some other variables called as control variables, where those variables will be kept constant in all the study participants. Example, in case of confounding variables, we keep those variables as a control variable. So, statistically, variables are broadly classified into categorical and numerical, under which we have nominal, ordinal, discrete interval ratio. So, down in the bottom, we have nominal, ordinal, interval ratio. We can remember it as NOIR, nominal, ordinal, interval and ratio. On the other hand, epidemiologically, we can have this independent variable, otherwise called as exposure variable, which determines or predicts the dependent variable or the outcome variable.