IB Physics IA: Design an Investigation You Can Defend
By Renzo Diaz·3 min read
The Physics scientific investigation is worth 20% of the final grade at both SL and HL, marked out of 24, with a 3,000-word limit on the report itself. The IB's own moderation guidance says the quantity of data collected should be appropriate to the investigation — not a fixed number of repeats — and the requirements are identical at SL and HL. Choosing a genuinely testable question and running a real pilot matters more than following an invented formula for "enough" data.
The Physics IA gets treated, by students and by some online advice, as a checklist to satisfy rather than an investigation to actually defend. That framing produces reports that hit invented targets — a specific number of data points, a specific number of repeats — without necessarily answering a real, testable question well. The IB's own guidance to examiners is narrower and more useful than most of the advice circulating about it.
The investigation is meant to be defensible, not merely compliant with an invented checklist.
1. What the investigation actually weighs and costs
The scientific investigation is worth 20% of the final subject grade, and the requirement is identical at SL and HL — there's no separate, easier version at standard level. It's marked by the student's own teacher first, out of 24, and then externally moderated by an IB examiner, who checks that the teacher's marking standard lines up with the standard set across all schools rather than remarking every piece of work from scratch.
The report itself has a hard limit: no more than 3,000 words. Data, raw calculations, and supporting information from the school don't count toward that limit when it's being marked — but the write-up, the reasoning, and the discussion do.
20% of the final grade, marked out of 24, capped at 3,000 words — and the requirement doesn't change between SL and HL.
2. There is no fixed formula for "enough" data
A specific, common piece of bad advice deserves a direct correction: there is no IB rule requiring five values with five repeats each, or any other fixed count, and no automatic mark penalty for using fewer. The IB's actual guidance to examiners moderating the investigation states plainly that the quantity of data collected or used should be appropriate to the investigation being carried out — a judgement call tied to what the specific investigation actually needs, not a number to hit regardless of context.
That distinction matters practically. An investigation built around a quick, high-precision measurement might genuinely need fewer repeats than one built around a noisy, hard-to-control variable — chasing a fixed repeat count in either case produces either wasted effort or a still-inadequate data set, depending on which direction the mismatch runs. The better question to ask before collecting any data is: what quantity and type of data would actually let me draw a defensible conclusion about this specific question?
The real standard is "appropriate to the investigation," not a fixed count of measurements and repeats.
3. Framing a testable question before collecting anything
A strong investigation starts from a question specific enough that a clear result — in either direction — would actually mean something, rather than a broad topic dressed up as a question. "How does temperature affect resistance" is a topic; "how does the resistance of a specific nichrome wire change as temperature increases from 20°C to 80°C, and does that relationship stay linear across the full range" is a question with a real, checkable answer.
Running a short pilot before committing to the full investigation catches problems a topic-level plan doesn't reveal — equipment that can't actually achieve the needed precision, a variable that's harder to control than expected, or a range too narrow to show a meaningful trend. A pilot that surfaces a problem before the real data collection begins is time well spent, not a delay.
4. Ask for the rubric your school is actually using
Different sources online describe the assessment criteria differently, and getting this wrong before writing wastes real effort in the wrong direction. Rather than relying on a secondhand summary of the criteria, ask your own school directly for the current Physics guide's assessment criteria — the authoritative version is the one your school's own IB coordinator or physics teacher has, tied to your specific cohort and exam session.
That single check — confirming which criteria actually apply, in their exact current wording, before structuring the report around them — is a five-minute conversation that prevents a much larger problem: a well-written investigation structured against the wrong rubric.
Sources
The source below is the IB's own examiner-facing documentation for the current assessment cycle, checked directly in a browser.