The IB's scientific-investigation model applies to Biology at SL and HL. The official 2025 examiner instructions describe an individually chosen investigation, up to 3,000 words, with a mark out of 24. This article focuses on reproducibility and honest method development, not on promising a particular criterion mark.
A complete pilot decision, with simulated measurements
Imagine an experiment estimating how temperature affects catalase activity using foam-height change during a fixed interval. Foam height is an imperfect proxy for oxygen release because bubble stability can vary, so that limitation must appear in the evaluation. The following observations are entirely synthetic teaching data, not results from a real laboratory.
An initial pilot at 25°C tests three response times using identical reagent volumes:
| Time after mixing | Replicate 1 | Replicate 2 | Replicate 3 | Mean foam height |
|---|---|---|---|---|
| 15 seconds | 8 mm | 9 mm | 7 mm | 8 mm |
| 30 seconds | 17 mm | 18 mm | 16 mm | 17 mm |
| 60 seconds | 22 mm | 23 mm | 21 mm | 22 mm |
The additional foam growth between 30 and 60 seconds is 5 mm, compared with 9 mm between 15 and 30 seconds. That does not prove enzyme activity has stopped. It suggests the measured proxy is approaching a different response regime. You could choose a fixed 30-second endpoint to preserve responsiveness or switch to a gas syringe to measure gas volume more directly. Choose based on what the apparatus can measure reliably, not which option gives a prettier graph.
The five-line pilot decision record
Question: Does the 60-second reading preserve enough sensitivity for comparing temperatures? Observation: In this simulated pilot, the average changes by 9 mm from 15–30 seconds but 5 mm from 30–60 seconds. Competing explanations: The reaction might slow, substrate might become limiting, bubbles might collapse, or the foam-height proxy might saturate. Decision: Use repeated 30-second measurements with mixing time tightly controlled; test the proxy against gas volume if equipment allows. Residual uncertainty: Foam-height readings still depend on container geometry and bubble persistence, and 30 seconds may not be optimal at every tested temperature.
That is a completed decision trail, not a decorative planning checklist. It also shows the difference between an empirical observation, a hypothesis about its mechanism, and a method revision.
A pilot can reveal false precision
Suppose your ruler has 1 mm divisions, the foam boundary is irregular, and two people disagree about whether a reading is 17 or 19 mm. Reporting 17.000 mm implies precision the instrument cannot support. Decide who reads the foam, where the foam height is measured, and what uncertainty reasonably reflects the measurement procedure. Instrument resolution and variability between observers are different uncertainty sources.
Likewise, three trials that all read 17 mm do not guarantee low experimental uncertainty. If all three share the same timing bias, replication will not reveal it. Test the sensitivity to timing errors or try a different measurement method where feasible.
What to fix before main data collection
Check that the independent variable has a controlled, measured range and enough levels to make your question answerable; confirm the dependent measure changes detectably over the selected period; standardize volumes, substrate concentration, enzyme source and mixing; decide how many independent runs you can genuinely collect; and check the lab safety and teacher approval requirements. Record why each control matters for causal inference.
The cleanest IA method is not the one with no revisions. It is the one whose revisions have defensible reasons and are transparently documented. Do not remove inconvenient but valid observations or backdate a pilot after collecting the main results.
Primary sources and limitations
- IB Biology examiner instructions, scientific investigation and 3,000-word guidance (2025): https://ibpublishing.ibo.org/exinst/apps/exinst/index.html?chapter=1&doc=EX_instructions_2025_e&part=11
- IB current curriculum-updates index for matching your assessment session: https://ibo.org/university-admission/latest-curriculum-updates
All figures are synthetic. Catalase foam is used solely to demonstrate experimental reasoning; it is not presented as official specimen work or as a preferred IB method. Draft-checking tools are available through ibacalao.com.
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