IB Hypothesis Writing Guide | Structure & Examples for IA and EE
A well-crafted hypothesis is the foundation of any strong IA or EE, yet few students are taught a reliable framework for writing one. Master the prediction–rationale–variable structure and you'll be able to write examiner-ready hypotheses with confidence.
What Makes a High-Scoring IB Hypothesis?
A strong IB hypothesis is not simply a guess—it is a falsifiable, theory-grounded prediction that makes the logic of your investigation visible to the examiner from the very first paragraph. Before you write a single word of your Internal Assessment (IA) or Extended Essay (EE), you need three things locked in:
- A clear prediction linking cause and effect
- A scientific or theoretical rationale explaining why you expect that outcome
- Explicit identification of your independent and dependent variables
Get those three right and the rest of your introduction builds naturally around them.
What Is the Standard If–Then–Because Format?
The most widely recognised structure for a science hypothesis in IB is:
"If [independent variable changes in this way], then [dependent variable will respond in this way], because [underlying scientific principle or mechanism]."
This format works because it forces you to commit to a direction, name your variables, and justify your prediction in a single sentence. Examiners reading dozens of IAs can immediately see whether you understand the theory behind your investigation.
Breaking Down Each Component
| Component | What It Does | Weak Version | Stronger Version |
|---|---|---|---|
| If… (IV) | Names the independent variable and the change you are testing | "If I change the temperature…" | "If the temperature of the enzyme solution is increased from 20 °C to 60 °C in 10 °C increments…" |
| Then… (DV) | States the predicted outcome with direction | "…then the reaction will be affected…" | "…then the rate of reaction, measured as oxygen production per minute, will increase and then decrease…" |
| Because… (rationale) | Links the prediction to theory or prior research | "…because heat affects enzymes." | "…because higher kinetic energy increases enzyme–substrate collisions until thermal denaturation of the active site disrupts binding above the optimum temperature." |
Notice how specificity in each clause does not require you to predict exact numbers—it requires you to show mechanistic thinking.
How Should a Hypothesis Differ Across Science Subjects?
IB science subjects share the same broad expectation—falsifiable prediction plus theoretical justification—but the vocabulary, depth of mechanism, and reference style vary. Always calibrate against the specific subject guide for your current syllabus session.
Biology
Biology hypotheses often involve biological mechanisms at the molecular or cellular level. A well-written Biology hypothesis names the enzyme, substrate, or organism, references the relevant pathway or system (e.g., enzyme–substrate specificity, osmoregulation, photosynthetic light reactions), and acknowledges likely confounding factors such as pH or co-factors.
Example framing: "If the concentration of amylase is increased while substrate (starch) concentration is held constant, then the rate of starch hydrolysis will increase up to a plateau, because at low enzyme concentrations the substrate is in excess and additional enzyme molecules provide more active sites for catalysis; however, once enzyme concentration matches substrate availability, the reaction rate will be limited by substrate, not enzyme."
For more on how examiners assess scientific understanding in Biology, see the IB Biology HL complete guide.
Chemistry
Chemistry hypotheses benefit from quantitative framing where the theory supports it. Reference specific laws (e.g., Le Chatelier's Principle, Beer–Lambert Law, collision theory) and explain why the variable you chose has a theoretically predictable effect on the outcome.
Example framing: "If the concentration of sodium thiosulfate solution is increased, then the rate of reaction with hydrochloric acid—measured as the time for a cross to become obscured—will increase (reaction time will decrease), because a higher concentration of reactant particles increases the frequency of successful collisions per unit volume, consistent with collision theory."
See the IB Chemistry HL complete guide for guidance on the level of theoretical detail expected across different topics.
Physics
Physics hypotheses often involve mathematical relationships. If your theory predicts a linear, inverse, or power-law relationship, say so explicitly in the hypothesis rather than leaving it for the analysis section.
Example framing: "If the length of a pendulum is increased, then its period of oscillation will increase in a non-linear (square-root) relationship, because according to the simple pendulum model T ∝ √L, a longer pendulum arm reduces the restoring force per unit displacement, resulting in a slower oscillation."
Psychology
Psychology hypotheses typically follow an experimental or quasi-experimental format and should reference a named theory, study, or cognitive/biological mechanism. The language shifts slightly—"participants," "scores," "conditions"—but the logical structure remains the same.
Example framing: "If participants encode a word list under conditions of semantic processing (deep encoding) rather than structural processing (shallow encoding), then recall scores on a surprise recognition test will be significantly higher in the deep-encoding condition, because Craik and Lockhart's levels-of-processing model proposes that elaborative rehearsal creates stronger and more durable memory traces than maintenance rehearsal."
What Are the Most Common Hypothesis Mistakes in IAs and EEs?
Even strong students trip over the same recurring errors. Here is a checklist of what to avoid:
How Do You Write a Hypothesis for an Extended Essay?
The Extended Essay (EE) raises the stakes because it is longer, more independently designed, and more heavily weighted on conceptual sophistication. For science EEs, the hypothesis functions as a through-line that the entire 4,000-word essay must answer. For humanities or social science EEs, the equivalent is often a research question with a stated direction of argument, but for experimental EEs in science subjects, a formal hypothesis is expected.
Specific considerations for EE hypotheses
- Situate it within existing literature. Unlike an IA where you reference syllabus theory, an EE hypothesis should acknowledge relevant published research or established models, even briefly.
- Be willing to predict a null result if theory supports it. If you are testing whether a variable has no significant effect under specific conditions, that is a legitimate and assessable hypothesis—as long as you justify it.
- Align your hypothesis with your research question. The two must logically connect: your research question asks, your hypothesis answers (tentatively), and your data either supports or refutes.
- Revisit the hypothesis in your conclusion. One of the clearest signals of a mature EE is a conclusion that explicitly returns to the hypothesis and evaluates the degree to which the data supports or contradicts it—including honest acknowledgement of uncertainty.
For a broader overview of EE structure and assessment, the IB Extended Essay complete guide covers the full process from topic selection through final submission.
How Do You Refine a Draft Hypothesis Before Finalising It?
Run your draft hypothesis through these four questions before you submit:
| Question | What You Are Checking |
|---|---|
| Can this prediction be proven wrong by data? | Falsifiability—if no possible result could contradict it, it is not a valid hypothesis |
| Does a reader know exactly what I am measuring? | Variable clarity—both IV and DV should be named and operationalised |
| Does my rationale cite a principle, model, or mechanism? | Theoretical depth—not just "because it makes sense" |
| Does the prediction specify a direction or relationship? | Directionality—increase, decrease, positive correlation, no significant difference, etc. |
If any answer is "no" or "not clearly," revise before moving on to your method.
A simple self-editing process
- Write your full hypothesis sentence.
- Underline the independent variable in one colour and the dependent variable in another.
- Circle the rationale clause beginning with "because."
- Read only the circled portion aloud. Does it introduce new information beyond restating the prediction? If not, deepen it.
- Ask a peer or teacher to identify your IV and DV from the hypothesis alone, without any other context. If they cannot, the sentence needs more precision.
A Quick Reference: Hypothesis Quality at a Glance
| Level | Characteristics | Example Fragment |
|---|---|---|
| Needs improvement | Vague, no direction, no mechanism | "Temperature will affect enzyme activity." |
| Developing | Has direction but weak rationale | "Higher temperature will increase enzyme activity because enzymes work better when warm." |
| Proficient | Direction + named mechanism | "Rate will increase because higher kinetic energy raises collision frequency between enzyme and substrate." |
| Strong | Direction + mechanism + boundary condition | "…rate will increase until denaturation of the active site above the optimum temperature reverses the trend." |
The jump from "developing" to "proficient" is primarily about naming the mechanism. The jump to "strong" is about knowing where your prediction's logic breaks down—and saying so.
Writing a rigorous hypothesis is one of the highest-leverage investments you can make in your IA or EE: it sharpens your method, guides your analysis, and signals to examiners that your investigation is theoretically grounded from the start. If you want personalised feedback on your specific hypothesis draft—particularly for complex topics where the theory is nuanced—working through it with an IB-experienced tutor at Quick IB can save you significant revision time later.