According to the publication manual of American Psychological Association (APA), cited sources are written in the text within parentheses with information about the author and year. APA is one of the most developed and used styles among those that originate from the so-called Harvard system. The style is frequently used in the natural sciences, social and behavioral sciences and medicine.
The basic idea behind a reference is that it should include enough information about the source so that the reader can easily find it. Therefore it is necessary to write your references in a consistent way.