The investigator is faced with one of the most difficult problems of obtaining or gathering the desired information or data. Utmost care must be exercised while collecting data because they constitute the foundation on which the superstructure of statistical analysis is built. The results obtained from the analysis are properly interpreted and policy decisions are taken. Hence, if the data are inaccurate and inadequate the whole analysis may be faulty and the decisions taken misleading.
Data may be obtained either from the primary source or the secondary source. A primary source is one that itself collects the data; a secondary source is one that makes available data which were collected by some other agency. For example, the data collected by the Ministry of Industries and made available through various publications constitute primary source. However, if the Ministry of Industries uses data collected by some other organization, say, National Sample Survery Organisation, this will constitute secondary source for the Ministry. A primary source usually has more detailed information particularly on the procedures followed in collecting and compiling the data. It may be noted that a given source may be partly primary and partly secondary. The journal ‘Agricultural Situation in India’ gives data compiled by the Ministry of Agriculture and Irrigation and may also contain related information collected by other Ministries.
It is preferable to make use of the primary source wherever possible for the following reasons:
(i) The secondary source may contain mistakes due to errors in transcription made when the figures were copied from the primary source
(ii) The primary source frequently includes definitions of terms and units used.
(iii) The primary source often includes a copy of the schedule and a description of the procedure used in selecting the sample and in collecting the data.
(iv) Primary source usually shows data in greater detail.
Depending on the source, statistical data are classified under two categories:
(i) Primary data, and
(ii) Secondary data.