Why using secondary data ?
Using data collected by others researchers is very interesting for your research.
1. You may want to find data similar to yours, but collected in other places/time period (comparison, discussion).
2. You may want to provide data in your research project proposal.
3. You may want to give information in a paper, without collecting data yourself.
How to do it ?
Check to see if there are any existing data that you can reuse, for examples by consulting relevant repositories.
You may find data via several ways, For example:
1. EOSC (Europen Open Science Cloud) lists repositories, via which you can find data
2. You can find datasets via Mendeley Data* website
3. OpenAire has a browser for all reasearch results and data collected in an research project financed by the European Commission
4. re3data.org. This website is dedicated to data repository searches. You can enter you research topic subject on the browser, and the add appropriate filter (options on the left) or browse results by country
5. Zanran.com is a search engine specially dedicated to find statistics, tables and figures related to a research topic. You can filter you results by place, date and type of file (excel, pdf, etc.). If you want to dowload them though, you will have to register (for free) first.
6. Datasearch.elsevier.com* is a search engine like scopus, but is specifically dedicated to find the data and tables of research papers.
7. Google* recently developed a browser to search for existing datasets. But you can also use the general browser with some boolean request like:
[research topic] + (database OR registry OR dataset OR statistics)
* Use of theses commercials sources are not validated by authorities