📚 Statistical Methods: Core Concepts & Exam Applications | 数学统计方法核心概念与考点应用
Statistics is one of the most accessible yet frequently underestimated topics in A-Level Mathematics. A clear grasp of core statistical methods — from measures of centre and spread to probability distributions and hypothesis testing — will allow you to solve exam questions quickly and confidently while avoiding costly marks lost to careless errors.
统计学是 A-Level 数学中最容易得分但也最常被低估的板块之一。透彻掌握统计方法的核心概念——从集中趋势与离散度量、概率分布到假设检验——能帮助你在考场上快速且自信地解题，避免因粗心失误而丢掉宝贵分数。
1. Measures of Central Tendency | 集中趋势度量
The three principal measures of central tendency are the mean, median and mode. The mean is calculated as Σx/n for raw data, or Σfx/Σf for grouped data, where f is the frequency of each class.
集中趋势的三个主要度量是均值（mean）、中位数（median）和众数（mode）。原始数据的均值为 Σx/n；分组数据的均值为 Σfx/Σf，其中 f 为各组的频数。
The median is the middle value when data are arranged in ascending order. For n observations, the median is the (n + 1)/2-th value; for grouped data, locate the class containing the cumulative frequency n/2 and interpolate.
中位数是将数据按升序排列后的中间值。对于 n 个观测值，中位数为第 (n + 1)/2 个数值；对于分组数据，需找到累积频数达到 n/2 的组，再进行插值估计。
The mode is the most frequently occurring value. In a grouped frequency table, the modal class is the class with the highest frequency density.
众数是出现频率最高的数值。在分组频数表中，众数所在组是频数密度最高的组。
In exams, you should choose the most appropriate measure: the mean uses all data but is sensitive to outliers; the median is robust to outliers; the mode is useful for categorical data.
在考试中，你需要选择最合适的度量：均值利用全部数据但对极端值敏感；中位数不受极端值影响；众数适用于分类数据。
2. Measures of Dispersion | 离散程度度量
Dispersion describes how spread out the data are. The simplest measures are the range and the interquartile range (IQR). The IQR is given by Q₃ − Q₁, where Q₁ is the lower quartile and Q₃ is the upper quartile.
离散程度描述数据的分散情况。最简单的度量是极差（range）和四分位距（IQR）。四分位距为 Q₃ − Q₁，其中 Q₁ 为下四分位数，Q₃ 为上四分位数。
The variance of a sample is defined as Σ(x − x̄)²/n, which simplifies to the computational form Σx²/n − x̄². The standard deviation σ is the positive square root of the variance.
样本方差定义为 Σ(x − x̄)²/n，计算时可化简为 Σx²/n − x̄²。标准差 σ 是方差的算术平方根（取正值）。
σ² = Σ(x − x̄)²/n = (Σx²/n) − x̄²
For grouped data, variance is computed as Σfx²/Σf − (Σ
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