There are three types based upon the number of variables studied as
a) Simple Correlation
b) Multiple Correlation
c) Partial Correlation
Simple Correlation:
If only two variables are taken for study then it is said to be simple correlation. Ex. Y= a + bx
Multiple Correlations:
If three or more than three variables are studied simultaneously, then it is termed as multiple correlation.
Ex: Determinants of Quantity demanded
Qd = f (P, Pc, Ps, t, y)
Where Qd stands for Quantity demanded, f stands for function.
P is the price of the goods,
Pc is the price of competitive goods
Ps is the price of substituting goods
t is the taste and preference
y is the income.
Partial Correlation:
If there are more than two variables but only two variables are considered keeping the other variables constant, then the correlation is said to be Partial Correlation

The Programme Implementation Wing has three Divisions, namely,
(i) Twenty Point Programme
(ii) Infrastructure Monitoring and Project Monitoring
(iii) Member of Parliament Local Area Development Scheme.

National Sample Survey Organisation (NSSO)
i. The National Sample Survey Organisation, now known as National Sample Survey Office, is an organization under the Ministry of Statistic of the Government of India.
ii. It is the largest organisation in India, conducting regular socio-economic surveys.

1. The Ministry of Statistics and Programme Implementation (MOSPI) came into existence as an Independent Ministry in 1999 after the merging of the Department of Statistics and the Department of Programme Implementation.
2. The Ministry has two wings, Statistics and Programme Implementation. The Statistics Wing called the National Statistical Office (NSO) consists of the Central Statistical Office (CSO), the Computer Centre and the National Sample Survey Office (NSSO).

1. Statistics is not suitable to the study of qualitative phenomenon:
Since statistics is basically a science and deals with a set of numerical data. It is applicable to the study of quantitative measurements.
2. Statistical laws are not exact:
It is well known that mathematical and physical sciences are exact. But statistical laws are not exact and statistical laws are only approximations.
3. Statistics table may be misused:
Statistics must be used only by experts; otherwise, statistical methods are the most dangerous tools on the hands of the inexpert.
4. Statistics is only one of the methods of studying a problem:
Statistical method does not provide complete solution of the problems because problems are to be studied taking the background of the countries culture, philosophy, religion etc., into consideration.