Statistics deals with data. Data is plural form of datum, which means any singular measure. Data are figures, ratings, check-lists and other information collected in experiments and surveys. Small data can be interpreted very easily. But it is not that easy to interpret a large data or a very large data. School children may be put in 1, 2, 3 order for height, marks on an examination or regularity of school attendance. Rank order gives us serial position in the group, but it does not provide an exact measurement.

Statistics is a science of variables; Characteristics of individuals permit them to be grouped in different classes or categories. All the individuals in one class may have the same value or trait. Characteristics having the same value are called constant. It has different values for different individuals, it is called a variable. In the study of graphs, we always come across two variables, the dependent and independent variables. There are two broad classes of variables; quantitative, which varies in amount and qualitative, varying in kind or quality.

A set of numerical facts obtained in the observation of a single variable is called a statistical series. When the items vary in size, they are capable of being classified in order of size.

The numerical data in statistical work may be classified as either continuous or discrete (discontinuous.) Continuous data arise from the measurement of continuous variables. An attribute or trait or characteristic is continuous if the values of its items differ by amounts which are indefinitely small such as age, test score etc. So in a continuous series there can be fractions. I.Q^{S}. for example, are usually thought of as increasing by increments of one unit along an ability continuum which runs forms idiot to genius. We might get I.Q^{S }of 100.8 or even 100.83. Physical measures as well as mental test scores fall into continuous series. The height of a child may be 160 cms. 160.3 cms, or 160.35 cms. Continuous statistical series are always quantitative, but all quantitative series are not continuous. Series which exhibit real gaps are called discrete. Discrete data are usually expressed in whole numbers'(integers). School enrolments, number of books in various libraries, census enumerations are examples of discrete data. In education mainly we use continues series.