Secondary data in research consists of several sources. Sometimes, secondary data is the only option because primary data is not available or difficult to reach.

Secondary data is called secondary because it is already manipulated. But, there are ways a researcher can use to improve the reliability and credibility of the secondary data in research.
For example, in qualitative research the researcher uses several tools to improve the credibility of the secondary sources.

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Authenticity: the researcher can check the authenticity of the source with regard to its soundness of material.
Credibility: the researcher can check the documents sources. Also check how the author evaluated the results in the document. This helps understand that the author did not distort the data.

Representativeness: the representativeness of the data in itself shows .
Meaning: concerned with how well the researcher will be able to understand the document.

Published data
Published Data is the most basic secondary source of information for data collection. The researcher can obtain published data from various sources like books, magazines, newspapers, journals and periodicals etc.
Published data is the most reliable secondary source of information. The validity of published data is greater than unpublished data.

Libraries and archives contain published records. Libraries carry a vast variety of books, journals and periodicals. You can use this information as a reference in your thesis, dissertation or other research articles. Periodicals and journals provide up to date information and they are also available in libraries. In social sciences and humanities newspapers can also be used to get references. Newspaper are an excellent source of data collection historical research. Newspaper are unique source of information for a researcher.Because, they contain some information that is not available in books or journals. The researcher can search in the archives for the old newspapers. But overall books and journals are easier to use because of the bibliography.

Personal records
Personnel records can also be very useful in research if data is unavailable in published from. Some personal sources like letters and diaries are invaluable assets. Letters and diaries can have a personal bias as they are written by human beings and they contain personal judgement. The researcher should eliminate any chances of bias in the source before using it in the research.

Electronic data
Electronic Data like movies, documentaries and television programs are also good sources for recording data in secondary research. In the social sciences and humanities they provide good source of information.
Government records
Government Records are available in the form of government surveys, tax records, census data and other statistical reports. They are easily available and widely used in research studies.
Public Sector Records
Public Sector Records are available in NGOs as well as some other public sector organization keeps records. They are present in government records in published or unpublished form.
Internet
Internet in today’s world is the fastest growing source of information. The internet has become mature and today you can get any information from the internet. Most of the books are available on the internet in e-book format. You can get information while staying at home. Internet is faster and easier source of information. On the internet you can get e-books, e-journals, e-periodicals and e-magazines. Today most books,magazines, journals, and newspapers are available on the internet. Getting information from the internet is inexpensive as compared to other sources. You can get membership of different periodicals, sometimes this membership is free otherwise you have to pay a small amount of money.