The distinction between primary, secondary and tertiary sources varies according to the discipline you are working in. A primary source in science is very different from one in history. Nonetheless, understanding the basic difference between these three types of sources and how they can be used will result in better quality production research.
Primary sources have a direct connection to the period you are researching, whether it is the world within the play, the world surrounding the play, or the period you have chosen to set your production in. These sources were either produced at the time or by people who were involved in the events. Some examples include:
These kinds of materials will help you gain a connection to the world of the play and its author and cultivate an original creative approach from a standpoint of deep understanding.
Secondary sources interpret, analyze, and contextualize a given subject. They usually involve analysis of primary sources, such as literary texts or historical documents. They will help you understand the context that you need to interpret primary source materials, and gain insight into the existing scholarly conversation on a given play and its author.
For academic research, the best practice is to draw primarily on scholarly journals and books published by academic presses. Although they can vary in quality, they generally are peer-reviewed and thus are vetted by other experts in the field before publication. Typical secondary sources include:
Tertiary sources offer a summary of existing knowledge on a topic. Some examples include:
These kinds of sources are valuable as a source of beginning-level, background knowledge on a topic. They may include lists of works cited or recommendations for further reading that can help you do more in-depth research and gain a deeper understanding of a topic.
For scholarly research, citing tertiary sources is usually discouraged except in limited circumstances. This is because you expected to develop more than a beginning-level knowledge of your research topic, and tertiary sources usually remain at a superficial level. You may miss out on important nuances and complexities as a result.
Also, tertiary sources may repeat widely held assumptions that have have been questioned or disproven in more carefully researched recent scholarship.