How to Write a Lab Report Discussion: The Four Moves After the Results

What the discussion is for (and what it is not)

The discussion answers the only question the report exists to answer: what do these results mean? Everything else in the report — methods, results, even the introduction — is scaffolding for this section. Graders read it first when time is short and grade it heaviest always, because interpretation is the skill the lab exists to build. A discussion that restates the results section in different words earns the comment 'summarizes rather than interprets,' which is the most common single deduction in introductory lab rubrics.

The boundary with results is mechanical and worth memorizing: results show, discussion tells. In the results section you present the table, the calibration curve, the mean and the percent recovery, with no interpretation beyond a figure caption. The discussion then says what the numbers mean, whether they were expected, why they were not, and what should happen next. Numbers reappear in the discussion — they must, to be interpreted — but as evidence inside sentences, never as naked repeats of the tables.

The numbers at a glance

Discussion length and required elements by course setting — the range, and where each row actually shows up in coursework. Skim it first; the sections below explain why the range exists and which row applies to you.

The four moves, in order

Nearly every strong discussion runs the same four moves in the same order. First, state the finding in one or two sentences with the key number attached: 'The aspirin sample assayed at 341 mg per tablet, 4.6% below the labeled 357 mg.' Next, compare it against expectation — the published value, the theoretical yield, the class average — quantified as percent difference or percent error. Then explain the discrepancy with specific, mechanistic error sources rather than a general apology. Finally, state the implication: does the hypothesis stand, and what would the next trial change?

The order matters because graders score moves by name even when the rubric phrases them as 'interpretation,' 'comparison to literature,' 'error analysis' and 'conclusion.' Skipping move two is the most frequent structural miss — students leap from their number straight to 'human error' without ever naming the accepted value the number missed. One paragraph per move works at introductory length; upper-division reports expand moves two and three into full paragraphs and let moves one and four share a closing one. Name the moves in the same words the rubric uses, so the grader finds each one without hunting.

How long the discussion should be

The proportions hold steady even when absolute lengths shift: the discussion typically runs 25-35% of the report's body, second only to results. Introductory chemistry and physics labs expect 150-250 words, general biology 200-300, and upper-division analytical or field labs 400-600, where quality control and detection limits need real explanation. When a rubric gives no length, estimate from the results: one interpretive sentence per major result, plus one comparison sentence and one error paragraph, lands most discussions in the right band automatically.

Length violations read differently above and below the band. An undersized discussion usually means moves two through four are missing, not that the writing is concise — check for the comparison sentence and the hypothesis verdict before assuming brevity was acceptable. An oversized discussion almost always contains smuggled results or methods narration, both of which belong elsewhere; cutting those restores the balance without losing a single interpretive idea. Write the four moves first, then trim adjectives, and the count lands close to target on its own.

Error analysis without the blame paragraph

Error analysis is explanation, not confession, and the distinction is graded. Systematic error shifts every measurement the same direction — a mis-calibrated spectrophotometer, a titrant standardized at the wrong molarity — and it is diagnosed by pointing at the direction of your deviation: results 8% low across all trials implicate something that removed mass or volume consistently. Random error scatters readings around the mean and is addressed with statistics: the standard deviation, the confidence interval, the spread across trials. Naming which kind you have is the move that separates real analysis from filler.

'Human error' is the phrase graders underline with disapproval, because it names no mechanism and explains nothing. Replace it with the specific act and its direction of effect: 'two drops of indicator beyond the endpoint would bias the volume high, inflating the calculated concentration.' Every named error needs that mechanistic sentence — what slipped, which way the result moves as a result — and where possible a check: the percent difference the error could plausibly produce. Three mechanistic sentences outscore a paragraph of apology every time the rubric is read closely.

Voice, tense and the hypothesis verdict

The tense convention is consistent across the sciences: your results are past tense ('the absorbance decreased'), established science is present tense ('caffeine absorbs at 274 nm'), and implications can take either depending on distance from the data. Voice follows course norms — most chemistry and biology departments accept first person plural in the discussion even where methods stay passive, but check the lab manual, because norms differ by department and some instructors still prefer full passive. Consistency inside the section matters more than which choice you make.

End with the verdict stated plainly: 'The hypothesis was supported' or 'The hypothesis was not supported, because the measured rate exceeded the predicted value at every temperature.' Science does not prove hypotheses, so the word 'proven' costs credibility points in courses that flag it, and 'partially supported' is a legitimate verdict when one outcome matched and another did not — provided the split is specified. The closing sentence belongs to significance: what the finding changes about the next trial, the method, or the understanding the lab started with.

Run your own numbers before you submit

Guidelines are ranges; your document is a fact. Paste your current draft into the word counter below and the page estimate recalculates live for double-spaced and single-spaced formats — no account, nothing uploaded, the text never leaves your browser. Check the word count rather than the page display in your editor: word processors pad pages with headings, breaks and whitespace, and instructors check words.

If the count lands below your target, add analysis before adding sources: two more sentences of interpretation in your three weakest paragraphs usually beats one new summary paragraph. If it lands above, cut your most summarizing paragraph — the one that retells a source instead of using it. Both edits take ten minutes and both make the paper better while fixing the number.