Q: Do I include evidence from the experiment in the rationale?

The rationale is typically part of the introductory matter, either a part of the introduction or a separate heading, but coming definitely before the plan of the experiment. This means that the evidence is used for justifying the need for the experiment you are either proposing (but have not yet performed, as in a proposal for funding) or have carried out (and are therefore writing up the account of the experiment as a research paper for publication). In either situation, you should not include the results (or what you refer to as ‘evidence’) in the rationale. In a proposal, the possibility is automatically ruled out because the experiment is yet to happen. In a research paper, it is not logical because the results cannot be a part of the justification for having undertaken that piece of research. In other words, the rationale (also known as justification) is written before including the evidence.

This of course does not apply to evidence from any earlier experiment(s), whether carried out by you or by others. By all means, you may include the evidence from those experiments to strengthen your proposal or as part of the introduction in which you describe what has been done before and how what you did was somewhat different although based on or drawing on the prior research. This of course (and not referring to your own studies) would come from doing a literature search of the topic.

Hope that helps. For more help on writing the rationale and even the introduction, you may find the following resources useful.

• How to write the rationale for research?
• Can you give an example of the "rationale of a study"?
• How to write a strong introduction section that makes a great first impression [Handbook]

The last resource is a handbook on R Upskill, a platform from our sister brand offering learning programs for researchers/academics. R Upskill offers several courses, handbooks, and other materials for various aspects of research writing and even for your research career as a whole. For a limited period, all programs are available for free. So, do try it today!

For now though, all the best for writing the rationale and the paper!