Glossary: Introduction to Biology and Laboratory Rules

This chapter's terms in context

Chapter 1 introduces the vocabulary a student uses for the rest of both Form 4 and Form 5, because every experiment described later in the syllabus is reported using the same five terms: hypothesis, manipulated variable, responding variable, controlled variable and inference. None of these terms describes a living structure; instead, they describe the logical skeleton of a scientific investigation, and SPM Biology tests them as carefully in genetics and ecology questions as it does in this opening chapter.

A hypothesis is a testable, tentative statement written before data is collected, predicting a relationship between two variables, for example, 'the higher the temperature, the faster the rate of enzyme activity, up to an optimum'. It is not a question and it is not a conclusion; it is a prediction that the experiment is designed to check. Once an experiment is planned, three kinds of variable have to be identified precisely. The manipulated variable is the one factor the experimenter deliberately changes, such as temperature. The responding variable is what is measured as a result, such as the volume of gas produced. The controlled variable is everything else that is kept constant so that only the manipulated variable can explain any change in the responding variable, pH, substrate concentration, enzyme concentration and reaction time in an enzyme experiment, for instance. Students most often lose marks in this chapter by swapping manipulated and responding variables, or by naming only one controlled variable when a question expects two or three.

Inference is different from all four of the above: it is a logical explanation drawn from an observation, connecting what was seen to an underlying biological reason. If a plant wilts in strong sunlight, the observation is the wilting itself; the inference is that transpiration exceeded water uptake by the roots. Examiners distinguish sharply between 'observation' (what is seen, measured or recorded) and 'inference' (the reasoned explanation for it), and a common error is to write an inference when an observation is asked for, or the reverse.

The full scientific method taught in this chapter, identifying a problem, forming a hypothesis, planning a fair test, collecting and interpreting data, and drawing a conclusion, gives every one of these five terms a fixed place. A conclusion is not the same as an inference either: a conclusion answers the original hypothesis directly by stating whether it is accepted or rejected based on the data gathered, while an inference explains a single specific observation. Learning to use these words in the right sentence position, rather than interchangeably, is exactly what separates a full-mark Paper 3 answer from a partial one.

A typical Paper 3 question in this chapter presents a short experimental scenario, for example, testing how leaf size affects the rate of water loss, and asks a candidate to state the hypothesis, identify the three variables, and give one inference about an observed result, all drawn from the same few sentences of context. Losing marks here is rarely about not knowing an experiment exists; it is about misreading which variable in the passage was actually changed by the experimenter and which was only measured, or about slipping into inference language when the question specifically asks only for what was observed. A candidate who reads the scenario twice, once to identify the manipulated and responding variables, and a second time to note every factor that must be controlled, before writing a single word of the answer consistently scores higher than one who begins writing immediately, because the five terms in this chapter are graded on precision of wording as much as on correct understanding.