When you are assigned a lengthy lab report, it is important to include a conclusion paragraph to sum up your procedures and results for your reader. A conclusion restates your goals and methods, includes any final data and notes whether you were able to successfully answer the questions posed by your experiment. If well-written, your conclusion helps the reader extract all the important points of your report while noting any of your experiment's unforeseen results.
Restate the Experiment's Goals
Begin your conclusion by restating the goals of your experiment. If you began your report with an introductory paragraph, briefly restate what you said there. Note all objectives of your experiment: What question or questions were you seeking to answer? Also include a summary of any predictions that you made for your experiment's results.
For example, let's say you performed an experiment to determine the freezing point for samples of water with different concentrations of salt. You would state that your experiment's goal was to find the relationship between salt concentration and water's freezing point. You would also include your prediction of how the salt concentration would affect the freezing point, based on your previous knowledge of chemistry.
Describe Methods Used
Provide a brief summary of the methods you used in your experiment. This should not be a comprehensive list of all items used in the experiment; the full list should be included in the "methods" section of your lab report. Note the important tools and substances in your experiment, and any methods used to obtain data. In addition to the summary of methods, include a brief explanation of why you chose those methods to obtain your data.
Include and Analyze Final Data
The heart of your lab report focuses on the data from your experiments -- including all the data you obtained along with a detailed analysis of that data. Your conclusion should not restate all the data from your experiment, only note any final data you've determined from analysis. For instance, if analyzing the data from an experiment to determine the density of formaldehyde produced an average result of 8.12 x 10^2kg/m^3, you would include only this result, and not any individual measurements from the experiment.
Your conclusion should also provide a brief explanation of what the final data from your experiment indicates. Explain any trends in your data, and note whether any irregularities in the results brought up further questions. Also report any possible sources of error in your data and your analysis.
State Whether Your Experiment Succeeded
Finally, in your conclusion, examine the data based on your goals and predictions for the experiment. State whether the results of your experiment allowed you to answer the questions that you set out in the introduction. If you were successful, state so. If not, provide a possible explanation for why your experiment was unable to answer these questions, and suggest a method that could be used in another experiment to better answer them. Regardless of whether you were successful, state what you've learned from your experiment, and note which of your predictions for the experiment's results were true.