📚 Identifying Sources of Error and Improving Experiments | 识别误差来源并改进实验
In A-Level Biology, the ability to identify sources of error and suggest improvements is a core practical skill assessed by CIE. Errors in experiments are broadly divided into random errors, which scatter results unpredictably around a true value, and systematic errors, which consistently shift results in one direction. Understanding this distinction is the first step toward designing better experiments and achieving more reliable data.
在 A-Level 生物学中，识别误差来源并提出改进建议是 CIE 考核的一项核心实验技能。实验中的误差大致分为两类：随机误差使结果围绕真实值不可预测地分散，而系统误差使结果一致地偏向某一方向。理解这一区别是设计更优实验、获得更可靠数据的第一步。
1. Categories of Error Sources | 误差来源的分类
Error sources in biological experiments can be grouped into four categories:
生物实验中的误差来源可分为四大类：
Apparatus errors: instruments are imprecise, uncalibrated, or used outside their optimal range.
仪器误差：仪器不精确、未校准，或超出最佳使用范围。
Procedural errors: the method itself introduces bias, such as non-random sampling or incorrect timing.
操作流程误差：方法本身引入偏差，例如非随机取样或计时不准确。
Biological variation: living materials differ between replicates in mass, age, or metabolic state.
生物变异：活体材料在质量、年龄或代谢状态上存在个体差异。
Environmental errors: uncontrolled variables such as temperature, light, or humidity fluctuate during the experiment.
环境误差：温度、光照、湿度等未受控变量在实验过程中发生波动。
When you plan improvements, examine each category systematically. A common mistake in exams is to say 'repeat the experiment' without specifying what was wrong and how repeating will fix it.
在规划改进方案时，应系统地检查每一类误差。考试中常见的错误是只说"重复实验"，却不说明哪里出了问题、重复实验为何能解决该问题。
2. Distinguishing Accuracy and Precision | 区分准确度与精密度
Before discussing errors, you must be able to distinguish accuracy and precision. Accuracy describes how close a measurement is to the true value; a systematic error reduces accuracy. Precision describes how close repeated readings are to each other; a large random error reduces precision.
在讨论误差之前，必须先区分准确度与精密度。准确度描述测量值与真实值的接近程度；系统误差会降低准确度。精密度描述重复读数之间的接近程度；较大的随机误差会降低精密度。
Accurate and precise → ideal data | 准确且精密 → 理想数据
Accurate but imprecise → small samples or variable conditions | 准确但不精密 → 样本量过小或条件波动
Precise but inaccurate → uncalibrated instrument | 精密但不准确 → 仪器未校准
Neither accurate nor precise → multiple error sources | 既不准确也不精密 → 存在多重误差来源
In exams, 'improve accuracy' usually means reducing systematic error, while 'reduce scatter' means tackling random error. Choose your language carefully.
在考试中，"提高准确度"通常意味着减少系统误差，而"减少离散度"则意味着处理随机误差。措辞要准确。
3. Instrument and Calibration Errors | 仪器与校准误差
1 g. A pH meter should be calibrated with standard buffer solutions before use; if not, the readings will be systematically offset.
生物实验室中的仪器包括天平、pH 计、比色计、温度计和容量玻璃器皿。每种仪器都有相应不确定度。精度为 ±0.01 g 的天平比 ±0.1 g 的天平更精密。pH 计使用前必须用标准缓冲液校准；否则读数会产生系统性偏移。
Percentage uncertainty = (instrument uncertainty ÷ measured value) × 100%
百分比不确定度 = (仪器不确定度 ÷ 测量值) × 100%
To improve: choose an instrument whose uncertainty is small relative to the measured value, calibrate it, and take readings within its optimal range. For example, a 25 cm³ measuring cylinder has a smaller percentage uncertainty than a 100 cm³ cylinder when measuring 25 cm³ of liquid.
改进方法：选择不确定度相对于测量值较小的仪器，进行校准，并在其最佳量程内读数。例如，测量 25 cm³ 液体时，25 cm³ 量筒的百分比不确定度小于 100 cm³ 量筒。
4. Human Observer Errors | 观察者人为误差
Human errors include parallax when reading a meniscus, judging a colour-change endpoint, or pressing a stopwatch late. Parallax is reduced by reading the scale at eye level. Endpoint judgement can be improved by using a colorimeter to detect colour change objectively instead of relying on the eye. Timing errors are minimised by using a data logger or by starting the timer at the moment of mixing rather than after.
人为误差包括读取弯月面时的视差、判断颜色变化终点的主观性，以及按下秒表的时间延迟。视差可通过平视刻度读数来减小。终点判断可通过比
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