Is primary source more reliable than secondary source in case of collecting data?
In most cases, a primary source is more valid. For example, say you want to learn about the life of Mostafa. There will be thousands of sources giving information about Mostafa, but they are only interpretations of his life given by people other than Mostafa that can be wrong or misleading. These are secondary sources because they did not come directly from Mostafa. However an autobiography written by Mostafa will most likely be more accurate and reliable because, after all, who knows Mostafa better than Mostafa? This is a primary source. It came directly from Mostafa and is not exposed to misinterpretation or misunderstanding.