The introduction sets the stage for the rest of your report. It should clearly outline the purpose of your study and why the reader should care about the results.
The most critical component of the introduction (and arguably the entire report) is a clearly stated specific purpose. If the reader needs to understand the study’s purpose so that they know what to expect in the results and the discussion. The purpose statement should be near the end of the introduction, usually in the last paragraph. It should be clearly stated in one or two sentences.
The purpose can be stated as a question you addressed or as a hypothesis you tested. For example, you might have conducted a study to examine how nitrogenous fertilizer affects the growth of a particular crop. In this situation, you might be interested in determining the optimum fertilizer application rate and also how the application rate might lead to toxicity. Thus you could frame the work as a question: what is the yield response of the particular crop to increasing nitrogen application rates? On the other hand, based on your reading of the literature you might have formulated a (testable) hypothesis: based on the crop cultivar and the soil type, maximal crop yield should occur at an application rate of 60 kg N per hectare, with visible toxicity occurring at applications above 90 kg N per hectare.
Having a clear understanding of your study's goal(s) is key to being able to write a quality research report.
The information leading up to the purpose statement should give biological context so that the reader can understand the stated purpose and why it was important to conduct the described study. Context may include defining new terms and concepts with which your audience might not be familiar and should summarize previously published results or observations that are directly related to the current study.
The background information must include a clear rationale for addressing the stated purpose. This rationale might be to further clarify previous results or to test a new hypothesis that arose from previous studies. The rationale is never solely to confirm previous results as all studies should aim to add new knowledge to the area of study.
The rationale for your study must be clear to the reader.
Usually, background information will come from primary literature and will need to be appropriately cited. When citing other scientist's work, make sure you are citing the primary source. If you found the information in another article's introduction, that article is NOT the primary source; you must read and cite the original work.
Note that at times it can be difficult to hunt down the original source; in this case, especially in the introduction, it is okay to cite a secondary source, but you must clarify for the reader that you did not read the original source. See Citing secondary sources.
Following the purpose statement, we usually include a brief description of the study design or approach. This description should not be more than 3 or 4 sentences. Do not include any method details because you will describe these in the methods section.
The purpose statement appears near the end of the introduction, and the introduction is usually written after the results and discussion are complete. Despite this, the purpose statement needs to be written early in the process because you need to have a good grasp of your study's purpose before you can meaningfully analyze the data collected. Additionally, having a clear understanding of your study's goal(s) is key to being able to write a quality research report. As you will see, both your results and discussion will be structured with the goal of specifically addressing the purpose stated in the introduction.
The lab manual may have spelled out the scientific purpose for the study you did in the lab. However, even in real life research, the purpose we set out to address is frequently not the purpose we end up addressing in our published papers. Do not be afraid to modify your purpose statement as you work through the process of writing your report.
Note: Your instructor may have spelled out learning goals for the lab; do not confuse these with the actual scientific purpose of the study.
Generally the introduction is written after the results and discussion are written because you must have a firm grasp on what your study shows and what it means before you can decide what background information your audience will need in order to understand the study and appreciate the results.
When deciding what background information to include, ask yourself, "Before I completed this study, what would I have needed to know in order to understand this study's purpose and what the results mean?"