1. It is defined as the positive square-root of the arithmetic mean of the square of the deviations of the given observation from their arithmetic mean.
2. The standard deviation of the population is denoted by the Greek letter σ (sigma) The standard deviation of sample is denoted as 's'.

1. Relative measures of dispersion are free from the units of measurements of the observations.
2. They are pure numbers
3. They are used to compare the variation in two or more sets, which are having different units of measurements of observations.

Absolute measure of dispersion indicates the amount of variation in a set of values in terms of units of observations.

The degree of variation is evaluated by various measures of dispersion. There are two kinds of measures of dispersion, namely
1. Absolute measure of dispersion
2. Relative measure of dispersion

1. Central value is called a measure of central tendency or an average or a measure of locations. There are five averages.
2. Among them mean, median and mode are called simple averages and the other two averages geometric mean and harmonic mean are called special averages.

(i) Primary data: Those data which do not already exist in any form, and thus have to be collected for the first time from the primary source(s). By their very nature, these data are fresh and first-time collected covering the whole population or a sample drawn from it
(ii) Secondary data: They already exist in some form: published or unpublished in an identifiable secondary source. They are, generally, available from published source(s), though not necessarily in the form actually required. Eg. Data from CSO, NSSO, RBI.

1. Rank data, on the other hand, are the result of assigning ranks to specify order in terms of the integers 1,2,3, ..., n.
2. Ranks may be assigned according to the level of performance in a test, a contest, a competition, an interview, or a show.

1. Nominal data are the outcome of classification into two or more categories of items or units comprising a sample or a population according to some quality characteristic.
2. Classification of students according to their sex (as males and females), Workers according to their skill (as skilled, semi-skilled, and unskilled), and of employees according to their level of education (as matriculates, undergraduates, and post-graduates).

1. Quantitative data are those that can be quantified in definite units of measurement. These refer to characteristics whose successive measurements yield quantifiable observations. Eg. Age, income, number of firms etc.
2. Qualitative data refer to qualitative characteristics of a subject or an object. A characteristic is qualitative in nature when its observations are defined and noted in terms of the presence or absence of a certain attribute in discrete numbers. These data are further classified as nominal and rank data. Eg. Gender, Community, honesty.

Data is the information about facts or numbers collected to be examined and used to help with decisions. Data are the basic raw materials of statistics.

1. In Medical sciences, statistical tools are widely used.
2. In order to test the efficiency of a new drug or to compare the efficiency of two drugs or two medicines, t - test for the two samples is used.
3. More and more applications of statistics are at present used in clinical investigation.

Statistical data and techniques are immensely useful in solving many economic problems such as fluctuation in wages, prices, production, distribution of income and wealth and so on.

1. Statistics is necessary for the formulation of policies to start new course, according to the changing environment.
2. There are many educational institutions owned by public and private engaged in research and development work to test the past knowledge and evolve new knowledge.
3. These are possible only through statistics.

1. Evidence from history proves that during the reign of Chandra Gupta Maurya, there existed a system of maintaining vital statistics, incuding registration of births and deaths.
2. Such records can be found in Kautilya’s Arthashastra even before 300B.C.
3. The book “Ain-e-Akbari” (1596- 97) mentions the statistical and administrative surveys conducted during Akbar’s rule.