📚 Experimental Design in Cambridge IGCSE Biology | 剑桥IGCSE生物学实验设计
Mastering experimental design is essential for high achievement in Cambridge IGCSE Biology. Whether you are tackling Paper 5 (Practical Test) or Paper 6 (Alternative to Practical), examiners expect you to plan fair investigations, handle variables, record data accurately, draw graphs, evaluate procedures and suggest meaningful improvements. This article walks you through every key step, explaining how to think like a scientist and earn full marks on experimental questions.
掌握实验设计对于在剑桥 IGCSE 生物考试中取得高分至关重要。无论是参加 Paper 5（实验操作考试）还是 Paper 6（实验替代笔试），考官都希望你能规划公平的探究、处理变量、精确记录数据、绘制图表、评估实验步骤并提出有意义的改进。本文将带你走过每一个关键步骤，解释如何像科学家一样思考，并在实验题中拿下满分。
1. Understanding Variables | 理解变量
In every experiment you must identify three types of variables. The independent variable is the factor you deliberately change. The dependent variable is what you measure, and the control variables are all the other conditions you keep the same to ensure a fair test.
在每一个实验中，你必须识别出三种变量。自变量是你刻意改变的因素。因变量是你测量的量，而控制变量是你为保障公平测试而保持恒定的所有其他条件。
For instance, when investigating the effect of temperature on amylase activity, the independent variable is the temperature of the water bath (e.g. 10, 20, 30, 40, 50 °C). The dependent variable could be the time taken for starch to disappear, tested by iodine solution. Control variables include pH, enzyme concentration, substrate volume, and the volume of iodine used for testing.
例如，在研究温度对淀粉酶活性影响的实验中，自变量是水浴温度（如 10、20、30、40、50 °C）。因变量可以是淀粉消失所需的时间，用碘液测试。控制变量包括 pH、酶浓度、底物体积和用于测试的碘液体积。
Only by changing one independent variable at a time can a student establish a clear cause‑and‑effect relationship. If two variables are altered simultaneously, you cannot tell which factor is responsible for any change in the results.
只有每次只改变一个自变量，学生才能建立起清晰的因果关系。如果同时改变两个变量，你就无法判断是哪个因素导致了结果的改变。
2. Writing a Testable Hypothesis | 撰写可检验的假设
A hypothesis is a precise, testable statement that predicts the outcome of an experiment. It usually follows the format: ‘If [independent variable changes], then [dependent variable changes] because [scientific reasoning].’
假设是一个精确、可检验的陈述，能够预测实验结果。它通常遵循如下格式：“如果[自变量改变]，那么[因变量改变]，因为[科学依据]。”
Example: ‘If the temperature increases from 10 °C to 40 °C, the time taken to completely digest starch will decrease, because enzyme molecules gain kinetic energy and collide more frequently with the substrate. Above the optimum, the enzyme denatures and the reaction slows.’
例子：“如果温度从 10 °C 升高到 40 °C，完全消化淀粉所需的时间将会减少，因为酶分子获得动能，与底物的碰撞更频繁。超过最适温度后，酶会变性，反应减慢。”
Always make the hypothesis specific and quantitative. Avoid vague words such as ‘faster’ or ‘better’ without measurable criteria. A strong hypothesis allows you to design an experiment that directly tests the prediction.
务必使假设具体且量化。避免使用没有可测量标准的模糊词语，如“更快”或“更好”。一个好的假设能让你设计出一个直接检验该预测的实验。
3. Designing a Fair Test | 设计公平测试
A fair test is one in which only the independent variable influences the dependent variable. All other conditions are kept strictly constant. This allows you to attribute any observed changes solely to the factor you are investigating.
公平测试是指只有自变量影响因变量，其他所有条件都被严格保持恒定。这样你便可将任何观察到的变化完全归因于你所研究的因素。
To carry out a fair test, use a control group or a baseline measurement where appropriate. For example, when testing the effect of pH on enzyme action, buffer solutions are used to maintain different pH levels, while temperature, enzyme concentration and substrate volume are kept identical in each tube.
为实施公平测试，在适当的情况下使用对照组或基线测量。例如，在测试 pH 对酶作用的影响时，使用缓冲液维持不同的 pH 值，同时保持各试管中的温度、酶浓度和底物体积完全相同。
Always repeat each measurement at least three times. Repeating improves the reliability of your data and allows you to calculate a mean, reducing the impact of random errors.
始终对每组测量至少重复三次。重复实验可以提高数据的可靠性，让你能计算出平均值，从而降低随机误差的影响。
4. Selecting Apparatus and Choosing Appropriate Equipment | 选择合适的器材与设备
Choosing the right apparatus can greatly improve accuracy. For approximate volumes, a measuring cylinder is acceptable, but for precise volumes you should use a volumetric pipette or a burette. A thermometer should have a scale fine enough to read to 0.5 °C or better.
选择合适的器材能极大提高准确性。对于近似体积，量筒是可以接受的，而要精确量取体积，则应使用移液管或滴定管。温度计的刻度应精确到 0.5 °C 或更佳。
When measuring time, use a stopwatch with a precision of 0.1 s for fast reactions. Thermostatically controlled water baths are essential when you need to maintain a constant temperature over a long period. Insulating beakers and using lids can also help minimise heat loss.
测量时间时，对于快速反应应使用精度为 0.1 秒的秒表。当需要长时间保持恒温时，恒温水浴是必不可少的。给烧杯保温并盖上盖子也有助于减少热量散失。
Always match the size of the instrument to the quantity being measured. For instance, to measure 25 cm³ of a liquid, a 25 cm³ pipette is far more accurate than a 100 cm³ measuring cylinder, because the percentage error is smaller.
始终使仪器的大小与所测量的量相匹配。例如，要量取 25 cm³ 液体，25 cm³ 移液管的准确度远高于 100 cm³ 量筒，因为百分误差更小。
5. Making Measurements and Observations | 进行测量与观察
Record all measurements immediately, using the correct units and an appropriate number of decimal places. Standard units in biology include °C for temperature, g for mass, cm³ for volume and s for time. Always read instruments at eye level to avoid parallax error.
立即记录所有测量值，使用正确的单位和适当的小数位数。生物学中的标准单位包括温度用 °C，质量用 g，体积用 cm³，时间用 s。始终在视线水平读取仪器，以避免视差误差。
Qualitative observations should be described clearly and objectively. Instead of writing ‘it changed colour’, write ‘the solution changed from blue-black to colourless’. When gas is produced, note whether bubbles are formed slowly, steadily or vigorously.
定性观察应清晰客观地描述。不要写“颜色变了”，而应写“溶液从蓝黑色变为无色”。当产生气体时，注意气泡是缓慢、平稳还是剧烈地生成。
If you take repeated readings, identify any anomalous results that deviate markedly from the general trend. These anomalies should be circled on a graph and excluded when calculating mean values, though you should comment on why they might have occurred.
如果你取得了重复读数，要识别出任何明显偏离总体趋势的异常结果。这些异常值在图表上应被圈出，并在计算平均值时排除，但你应该说明它们可能出现的原因。
6. Data Recording and Tables | 数据记录与表格设计
A well‑designed results table is vital. The header row must contain the variable name and its unit, separated by a forward slash (e.g. Temperature / °C, Time / s). The independent variable is placed in the first column, followed by columns for each repeat reading and the calculated mean.
设计良好的结果表格至关重要。表头行必须包含变量名及其单位，用斜线分隔（如 Temperature / °C，Time / s）。自变量放在第一列，随后是每次重复读数和计算出的平均值。
Below is an example of a results table for an experiment that tests how temperature affects the time for starch to be digested:
以下是一个实验结果表格的示例，实验测试了温度如何影响淀粉被消化的时间：
| Temperature / °C | Trial 1 / s | Trial 2 / s | Trial 3 / s | Mean / s |
|---|---|---|---|---|
| 10 | 125 | 118 | 122 | 122 |
| 20 | 78 | 82 | 74 | 78 |
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