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Descriptive statistics
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A descriptive statistic (in the count noun sense) is a summary statistic that quantitatively describes or summarizes features from a collection of information,[1] while descriptive statistics (in the mass noun sense) is the process of using and analysing those statistics. Descriptive statistics is distinguished from inferential statistics (or inductive statistics) by its aim to summarize a sample, rather than use the data to learn about the population that the sample of data is thought to represent.[2] This generally means that descriptive statistics, unlike inferential statistics, is not developed on the basis of probability theory, and are frequently nonparametric statistics.[3] Even when a data analysis draws its main conclusions using inferential statistics, descriptive statistics are generally also presented.[4] For example, in papers reporting on human subjects, typically a table is included giving the overall sample size, sample sizes in important subgroups (e.g., for each treatment or exposure group), and demographic or clinical characteristics such as the average age, the proportion of subjects of each sex, the proportion of subjects with related co-morbidities, etc.
Some measures that are commonly used to describe a data set are measures of central tendency and measures of variability or dispersion. Measures of central tendency include the mean, median and mode, while measures of variability include the standard deviation (or variance), the minimum and maximum values of the variables, kurtosis and skewness.[5]
Use in statistical analysis
[edit]Descriptive statistics provide simple summaries about the sample and about the observations that have been made. Such summaries may be either quantitative, i.e. summary statistics, or visual, i.e. simple-to-understand graphs. These summaries may either form the basis of the initial description of the data as part of a more extensive statistical analysis, or they may be sufficient in and of themselves for a particular investigation.
For example, the shooting percentage in basketball is a descriptive statistic that summarizes the performance of a player or a team. This number is the number of shots made divided by the number of shots taken. For example, a player who shoots 33% is making approximately one shot in every three. The percentage summarizes or describes multiple discrete events. Consider also the grade point average. This single number describes the general performance of a student across the range of their course experiences.[6]
The use of descriptive and summary statistics has an extensive history and, indeed, the simple tabulation of populations and of economic data was the first way the topic of statistics appeared. More recently, a collection of summarisation techniques has been formulated under the heading of exploratory data analysis: an example of such a technique is the box plot.
In the business world, descriptive statistics provides a useful summary of many types of data. For example, investors and brokers may use a historical account of return behaviour by performing empirical and analytical analyses on their investments in order to make better investing decisions in the future.
Univariate analysis
[edit]Univariate analysis involves describing the distribution of a single variable, including its central tendency (including the mean, median, and mode) and dispersion (including the range and quartiles of the data-set, and measures of spread such as the variance and standard deviation). The shape of the distribution may also be described via indices such as skewness and kurtosis. Characteristics of a variable's distribution may also be depicted in graphical or tabular format, including histograms and stem-and-leaf display.
Bivariate and multivariate analysis
[edit]When a sample consists of more than one variable, descriptive statistics may be used to describe the relationship between pairs of variables. In this case, descriptive statistics include:
- Cross-tabulations and contingency tables
- Graphical representation via scatterplots
- Quantitative measures of dependence
- Descriptions of conditional distributions
The main reason for differentiating univariate and bivariate analysis is that bivariate analysis is not only a simple descriptive analysis, but also it describes the relationship between two different variables.[7] Quantitative measures of dependence include correlation (such as Pearson's r when both variables are continuous, or Spearman's rho if one or both are not) and covariance (which reflects the scale variables are measured on). The slope, in regression analysis, also reflects the relationship between variables. The unstandardised slope indicates the unit change in the criterion variable for a one unit change in the predictor. The standardised slope indicates this change in standardised (z-score) units. Highly skewed data are often transformed by taking logarithms. The use of logarithms makes graphs more symmetrical and look more similar to the normal distribution, making them easier to interpret intuitively.[8]: 47
References
[edit]- ^ Mann, Prem S. (1995). Introductory Statistics (2nd ed.). Wiley. ISBN 0-471-31009-3.
- ^ Christopher, Andrew N. (2017), "Drawing Conclusions From Data: Descriptive Statistics, Inferential Statistics, and Hypothesis Testing", Interpreting and Using Statistics in Psychological Research, Thousand Oaks, CA: SAGE Publications, Inc, pp. 145–183, doi:10.4135/9781506304144.n6, ISBN 978-1-5063-0416-8, retrieved 2021-06-01
- ^ Dodge, Y. (2003). The Oxford Dictionary of Statistical Terms. OUP. ISBN 0-19-850994-4.
- ^ Christopher, Andrew N. (2017), "Drawing Conclusions From Data: Descriptive Statistics, Inferential Statistics, and Hypothesis Testing", Interpreting and Using Statistics in Psychological Research, Thousand Oaks, CA: SAGE Publications, Inc, pp. 145–183, doi:10.4135/9781506304144.n6, ISBN 978-1-5063-0416-8, retrieved 2021-06-01
- ^ Investopedia, Descriptive Statistics Terms
- ^ Trochim, William M. K. (2006). "Descriptive statistics". Research Methods Knowledge Base. Retrieved 14 March 2011.
- ^ Babbie, Earl R. (2009). The Practice of Social Research (12th ed.). Wadsworth. pp. 436–440. ISBN 978-0-495-59841-1.
- ^ Nick, Todd G. (2007). "Descriptive Statistics". Topics in Biostatistics. Methods in Molecular Biology. Vol. 404. New York: Springer. pp. 33–52. doi:10.1007/978-1-59745-530-5_3. ISBN 978-1-58829-531-6. PMID 18450044.
External links
[edit]- Descriptive Statistics Lecture: University of Pittsburgh Supercourse: http://www.pitt.edu/~super1/lecture/lec0421/index.htm
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Descriptive statistics
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Fundamentals
Definition and purpose
Descriptive statistics is the branch of statistics that involves the analysis, summarization, and presentation of data sets to describe their features through numerical measures, tables, or graphs.[1] This approach organizes raw data into a more comprehensible form, highlighting key characteristics such as the overall structure and composition of the dataset without attempting to draw conclusions beyond the data itself.[4] The primary purpose of descriptive statistics is to condense large or complex data sets into concise summaries that reveal patterns, trends, and essential features, facilitating easier interpretation and serving as a foundational step for subsequent analyses.[4] By focusing on the observed data, it enables researchers, analysts, and decision-makers to gain initial insights into variables like distributions or relationships, aiding fields from psychology to economics without inferring broader generalizations.[5] The development of descriptive statistics emerged in the 18th and 19th centuries, building on early ideas of averaging and graphical representation, with key advancements by Francis Galton in examining human traits through correlation and regression, and by Karl Pearson in introducing tools like the histogram for data visualization.[4][6] For instance, a simple dataset of heights from 10 individuals could be summarized by grouping values into height ranges and noting the count in each, transforming detailed individual measurements into a straightforward overview of the group's composition.[1] In contrast to inferential statistics, which extend findings to larger populations, descriptive methods remain confined to the dataset at hand.[5]Distinction from inferential statistics
Descriptive statistics focus on summarizing and organizing the observed data from a specific sample, providing exact descriptions such as the mean or frequency distributions without extending beyond the dataset itself.[7] In contrast, inferential statistics use that sample data to make probabilistic estimates about a broader population, incorporating tools like confidence intervals to quantify uncertainty in those estimates.[7] For example, reporting the exact average height of 100 surveyed individuals represents descriptive statistics, while using that average to infer the height of an entire community with a margin of error exemplifies inferential approaches.[8] The scope of descriptive statistics is inherently limited to exploration and confirmation within the collected data, emphasizing patterns and trends observable directly from the sample.[9] Inferential statistics, however, broaden this scope through hypothesis testing and generalization, enabling conclusions about population characteristics or relationships that were not directly measured.[9] An overlap occurs when a descriptive measure, such as a sample mean, transitions into an inferential context by serving as the basis for population estimation, highlighting how the two branches can complement each other in analysis.[7] Despite their utility, descriptive statistics cannot prove causation or support generalizations outside the sampled data, as they lack the probabilistic framework required for such extensions.[8] Inferential methods are thus essential to address these limitations, providing the rigor needed to draw reliable inferences from samples to populations.[7]Measures of Central Tendency
Arithmetic mean
The arithmetic mean, often simply called the mean, is a fundamental measure of central tendency in descriptive statistics, defined as the sum of all data values divided by the number of observations.[10] It provides a single value that summarizes the "center" or typical value of a dataset, assuming equal importance for each observation.[11] The formula for the arithmetic mean of a dataset with values is given by: This expression arises from the basic arithmetic operation of averaging, where the total sum is evenly distributed across the observations.[10] To calculate the arithmetic mean, first sum all the values in the dataset and then divide by the total number of values. For example, consider a small dataset of test scores: 80, 90, and 100. The sum is 270, and with three scores, the mean is .[11] This process can extend to larger datasets or grouped data using frequencies, such as where represents the frequency of each value .[10] Key properties of the arithmetic mean include its sensitivity to extreme values (outliers), which can disproportionately influence the result, especially in small samples; its additivity, meaning the mean of sums equals the sum of means for independent groups; and its utility in weighted averages, where observations are assigned different importance via weights in the formula .[10] Additionally, the sum of deviations from the mean equals zero, making it a balanced point for further statistical analysis. The arithmetic mean offers advantages such as incorporating every data point for a comprehensive representation, resisting random fluctuations across repeated samples, and serving as a foundation for other statistical measures like the standard deviation.[10] However, its disadvantages include vulnerability to skewing by outliers, rendering it less suitable for highly skewed distributions or non-numeric data, where alternatives like the median may provide a more robust central measure.[10][11]Median and mode
The median is a measure of central tendency that represents the middle value in a data set when the observations are arranged in ascending order.[13] For an odd number of observations , it is the value at position ; for an even number, it is the average of the values at positions and .[14] For example, in the income data set {10,000, 20,000, 50,000, 1,000,000}, sorted as {10,000, 20,000, 50,000, 1,000,000}, the median is the average of 20,000 and 50,000, which is 35,000.[15] Unlike the arithmetic mean, the median is robust to outliers because it depends only on the order of the data rather than their magnitudes.[16] It is particularly useful for skewed distributions, where extreme values might distort the mean, providing a better representation of the typical value.[17] The mode is the value or values that occur most frequently in a data set, serving as another measure of central tendency.[18] A data set is unimodal if it has one mode, bimodal if it has two, and multimodal if it has more than two.[19] For example, in the color preferences {red, red, blue}, the mode is red, as it appears twice while blue appears once.[20] The mode is especially valuable for nominal or categorical data, where arithmetic operations are not applicable, making it the only central tendency measure suitable for such variables.[21] It is commonly used to summarize the most common category in frequency distributions, such as preferred product types in market research.[22]Measures of Variability
Range and interquartile range
The range is a basic measure of statistical dispersion that quantifies the spread of data by calculating the difference between the maximum and minimum values in a dataset.[23] It is defined by the formula , where represents the data points.[18] For example, in a set of daily temperatures recorded as 20°C, 22°C, 25°C, 27°C, and 30°C, the range is 30°C - 20°C = 10°C, indicating the full extent of temperature variation observed.[18] While straightforward to compute and interpret, the range is highly sensitive to outliers, as a single extreme value can dramatically alter the maximum or minimum and thus the overall measure.[24] The interquartile range (IQR) provides a more robust alternative measure of spread by focusing on the middle 50% of the data, specifically the difference between the third quartile (Q3) and the first quartile (Q1).[25] Quartiles divide an ordered dataset into four equal parts: Q1 marks the 25th percentile (lower quartile), Q2 the median (50th percentile), and Q3 the 75th percentile (upper quartile).[24] The IQR is calculated using the formula .[25] To compute the IQR, first arrange the data in ascending order to form an ordered list.[25] Next, locate the median (Q2) by finding the middle value (or average of the two middle values if the dataset has an even number of observations), which splits the data into lower and upper halves.[25] Then, determine Q1 as the median of the lower half (excluding Q2 if n is odd) and Q3 as the median of the upper half, again averaging if necessary for even-sized halves.[24] Finally, subtract Q1 from Q3 to obtain the IQR; for instance, in the ordered dataset {1, 2, 3, 4, 5, 6, 7}, Q1 = 2, Q3 = 6, and IQR = 4.[25] Unlike the range, the IQR is resistant to the influence of outliers because it excludes the lowest 25% and highest 25% of the data, emphasizing central variability instead.[24] This robustness makes the IQR particularly useful in datasets with potential extremes, though it provides less information about the full data spread compared to measures like standard deviation.[25]Variance and standard deviation
Variance measures the average squared deviation of data points from the mean, providing a quantitative assessment of data dispersion. For a population of size with mean , the population variance is calculated as , where are the individual data values.[26] This formula averages the squared differences to emphasize larger deviations and yield a non-negative value.[27] When working with a sample of size drawn from a larger population, the sample variance uses the formula , where is the sample mean.[28] The denominator instead of applies Bessel's correction, which adjusts for the bias introduced by estimating the population mean from the sample, ensuring is an unbiased estimator of .[29] Without this correction, the sample variance would systematically underestimate the true population variance due to the sample mean minimizing deviations within the sample.[30] The standard deviation is the square root of the variance, returning the measure to the original scale of the data: population standard deviation and sample standard deviation .[31] This property makes the standard deviation more interpretable than variance, as it expresses dispersion in the same units as the data, roughly representing the average deviation from the mean.[32] For instance, in a dataset with a standard deviation of 10 units, typical values are expected to deviate by about 10 units from the mean.[33] A key property of the standard deviation is its sensitivity to outliers, as squaring deviations in the variance amplifies extreme values, and the coefficient of variation (CV) addresses relative dispersion by normalizing it: (often expressed as a percentage).[34] The CV allows comparison of variability across datasets with different units or scales, such as relative spread in incomes versus test scores.[35] Consider a sample dataset of test scores: 70, 80, 90, 100, 110. The sample mean is 90. The squared deviations from the mean are (70-90)^2 = 400, (80-90)^2 = 100, (90-90)^2 = 0, (100-90)^2 = 100, and (110-90)^2 = 400. Summing these gives 1000, and dividing by yields . Thus, the sample standard deviation , and the CV is , indicating moderate relative variability.[28]Measures of Distribution Shape
Skewness
Skewness is a measure of the asymmetry in the distribution of a dataset, quantifying the extent to which the tails on either side of the mean differ in length or weight.[36] A symmetric distribution, such as the normal distribution, has a skewness of zero, indicating equal balance on both sides. Positive skewness, or right-skewed distributions, occur when the right tail is longer or fatter, pulling the mean toward higher values; negative skewness, or left-skewed distributions, feature a longer or fatter left tail, shifting the mean leftward.[36] One common way to calculate skewness is Pearson's first skewness coefficient, defined as , where is the mean, mode is the most frequent value, and is the standard deviation; this mode-based measure highlights deviations from symmetry using central tendency.[37] An alternative, often called Pearson's second skewness coefficient, uses the median instead: , which is useful when the mode is ill-defined or multimodal.[37] These coefficients, introduced by Karl Pearson in his foundational work on frequency curves, provide simple, interpretable assessments of asymmetry without requiring higher moments.[38] A more formal approach is the moment-based skewness coefficient, or Fisher-Pearson standardized third moment, given for a sample by , where is the sample standard deviation (with in the denominator for consistency with population moments).[36] This formula captures the third central moment normalized by the cube of the standard deviation, yielding values greater than zero for right skew and less than zero for left skew. For interpretation, consider income distributions, which are typically right-skewed due to a concentration of lower incomes and a long tail of high earners; here, the mean exceeds the median, as extreme values inflate the average while the median better represents the typical earner.[39] In such cases, skewness values around 1 to 2 indicate moderate asymmetry, affecting choices in summary statistics where the mean may mislead compared to the median.[36]Kurtosis
Kurtosis quantifies the extent to which a probability distribution exhibits heavy or light tails and a peaked or flat central region compared to the normal distribution. Introduced by Karl Pearson in 1905, it originally assessed the "flat-toppedness" of symmetric distributions relative to the Gaussian curve, serving as the fourth standardized moment to describe shape beyond centrality and variability. In modern usage, kurtosis emphasizes tail extremity, where higher values indicate greater concentration of probability in the tails (heavy-tailed) and lower values suggest thinner tails (light-tailed).[36] The population kurtosis is the fourth central moment divided by the square of the variance: , where is the fourth central moment and is the variance; for the standard normal distribution, . Excess kurtosis, which centers the normal distribution at zero, is defined as . For samples, the bias-corrected estimator of excess kurtosis is where is the sample size, is the sample mean, and is the sample standard deviation; this adjustment accounts for finite-sample bias and is recommended for .[40] Distributions are classified by excess kurtosis into three types. Leptokurtic distributions () feature heavy tails and a pronounced peak, increasing the likelihood of extreme outliers relative to the normal. Platykurtic distributions () have light tails and a broader, flatter peak, with fewer extremes. Mesokurtic distributions () match the normal distribution's tail and peak characteristics. These terms, coined by Pearson, highlight kurtosis's role in capturing vertical shape aspects distinct from asymmetry.[41] In practice, leptokurtic distributions appear in financial returns, where extreme market events like crashes or booms create heavier tails than a normal model would predict, elevating risk assessments. Conversely, the uniform distribution exemplifies platykurtosis with its constant density, yielding an excess kurtosis of -1.2 and no propensity for outliers.[42][43]Graphical Methods
Histograms and frequency polygons
A histogram is a graphical representation of the distribution of numerical data, constructed by dividing the data range into intervals known as bins and displaying the frequency or relative frequency of observations within each bin as the height of adjacent bars.[44] The bars are contiguous without gaps, reflecting the continuous nature of the underlying data, and the x-axis represents the bin intervals while the y-axis shows the frequency counts.[44] To construct a histogram, first sort the data and define the bins by selecting the number of intervals and their width, ensuring coverage of the entire data range without overlap.[44] The choice of bin width is critical, as it influences the histogram's smoothness and detail; a common guideline is Sturges' rule, which recommends the number of bins , where is the sample size, rounded up to the nearest integer.[45] This rule works best for symmetric, normally distributed data but may oversmooth for large or skewed datasets.[45] Next, count the frequency of data points falling into each bin to create a frequency distribution table, then plot the bars with widths equal to the bin size and heights proportional to the frequencies.[44] For example, consider age data from a group of 90 band members ranging from 15 to 40 years, grouped into 5-year bins: 15–20 years (8 members), 20–25 years (14 members), 25–30 years (23 members), 30–35 years (29 members), and 35–40 years (16 members).[46] Sorting the ages and tallying frequencies into these bins yields the table, and plotting the corresponding bars produces a histogram peaking at the 30–35 bin, illustrating the age concentration.[46] A frequency polygon is a line graph that approximates the histogram by connecting the midpoints of each bin's top edge, providing a smoothed visualization of the frequency distribution's trends.[47] To construct it, mark the midpoint of each bin on the x-axis and the corresponding frequency on the y-axis, then join these points with straight lines; often, the polygon is closed by extending lines to zero-frequency points just outside the first and last bins.[47] For the age data example, midpoints would be at 17.5, 22.5, 27.5, 32.5, and 37.5 years, with lines connecting the points (17.5, 8), (22.5, 14), (27.5, 23), (32.5, 29), and (37.5, 16), revealing an upward trend to a peak followed by a decline.[46][48] Histograms and frequency polygons are used to reveal the overall shape of a data distribution, such as unimodal or bimodal patterns, identify clusters of high density, detect gaps or outliers, and assess symmetry or skewness.[44][47] Frequency polygons particularly facilitate trend visualization and comparison across multiple distributions by overlaying lines.[47] However, both tools are sensitive to binning choices, which can introduce artifacts like artificial peaks or smoothed features that misrepresent the true distribution, especially in small samples or with varying resolutions.[49][50] Misinterpretations arise if bin widths are too narrow, amplifying noise, or too wide, obscuring details.[51]Box plots and scatter plots
A box plot, also known as a box-and-whisker plot, is a standardized graphical representation of a dataset's distribution that summarizes key statistical measures including the median, quartiles, and potential outliers.[52] Introduced by John W. Tukey in his seminal work on exploratory data analysis, the box plot provides a compact visual summary that highlights the central tendency, spread, and asymmetry of data without displaying every individual point.[53] To construct a box plot, first calculate the five-number summary: the minimum value, the first quartile (Q1, at the 25th percentile), the median (Q2, at the 50th percentile), the third quartile (Q3, at the 75th percentile), and the maximum value. Draw a rectangular box extending from Q1 to Q3, with a horizontal line inside at the median; the height of this box represents the interquartile range (IQR = Q3 - Q1). Extend whiskers from the box edges to the smallest and largest values that fall within 1.5 times the IQR from Q1 and Q3, respectively; data points beyond these fences are plotted as individual outliers.[53][52] For example, consider a dataset of exam scores: if Q1 = 60, median = 75, Q3 = 85, and IQR = 25, the whiskers would extend to scores within 60 - 37.5 (22.5) and 85 + 37.5 (122.5), flagging any scores outside this range as outliers.[52] Box plots are particularly useful for illustrating the spread and variability of data within a single group or for comparing distributions across multiple groups, such as treatment versus control in an experiment.[52] They resist misrepresentation of data by focusing on robust summary statistics rather than sensitive measures like the mean, making them effective for skewed distributions.[54] One key advantage is their ability to quickly identify outliers and skewness, providing a rich yet compact visualization that facilitates comparisons without overwhelming detail.[55] A scatter plot is a graphical tool that displays the relationship between two continuous variables by plotting data points on a Cartesian plane, with one variable on the x-axis and the other on the y-axis.[56] Each point represents an observation's paired values, allowing visual assessment of patterns such as clustering, trends, or dispersion.[57] Construction involves selecting the two variables, scaling the axes appropriately, and marking a point at the intersection of each pair's values; no lines are initially drawn, emphasizing the raw bivariate distribution.[56] Trend lines, such as a linear regression line, can be added post-construction to quantify the association, with the slope indicating direction and strength.[56] For instance, plotting height against weight might reveal a positive linear pattern if points trend upward from left to right.[57] Scatter plots excel at revealing the strength of associations between variables, including positive, negative, or absent correlations, and are essential for detecting non-linear patterns or outliers in bivariate data.[57][56] Their primary advantage lies in visualizing complex relationships at a glance, such as curvature or clusters, which summary statistics alone might overlook, thus aiding exploratory analysis of potential dependencies.[56][58]Applications in Analysis
Univariate analysis
Univariate analysis in descriptive statistics focuses on summarizing and exploring the distribution of a single variable within a dataset to identify patterns, central tendencies, and variations. This approach allows researchers to gain initial insights into the data's characteristics without considering relationships with other variables.[59] It serves as a foundational step in data exploration, providing a clear picture of how values are distributed, which is essential for subsequent analyses.[60] The process typically involves calculating measures of central tendency, such as the mean, and measures of spread, like the standard deviation, alongside graphical depictions to describe the variable's distribution. For instance, the mean provides an average value, while the standard deviation quantifies the average deviation from that mean, revealing the data's variability.[61] Histograms are commonly used to visualize the frequency distribution, showing the shape, center, and spread through bars representing value ranges.[62] This combination helps in understanding whether the data clusters around certain values or exhibits wide dispersion. A practical example is analyzing exam scores from a class of students, where the mean score indicates overall performance and the standard deviation highlights the consistency or variability in achievement levels.[63] Similarly, for a dataset of adult heights, the mean and standard deviation summarize average stature and typical variation, while a histogram can reveal the distribution's shape, such as whether it approximates a symmetric bell curve.[60] Complementary tools include frequency tables, which organize data into categories or intervals to show counts of occurrences, aiding in the identification of common values.[64] Stem-and-leaf plots provide a textual graphical representation that retains exact data values while displaying the distribution's shape, useful for small to moderate datasets.[65] Through univariate analysis, analysts can detect anomalies, such as outliers that deviate markedly from the main cluster in a histogram, and assess normality by examining if the distribution appears symmetric and unimodal.[66] In modern computing environments, this is facilitated by software like R, where functions such asmean(), sd(), and hist() compute and plot these summaries efficiently, or Python's NumPy library for numpy.mean() and numpy.std(), paired with Matplotlib's plt.hist() for visualization.[67][68] These tools streamline the process, enabling quick pattern recognition in large datasets. Univariate analysis lays the groundwork for bivariate extensions that examine variable relationships.[62]
Bivariate and multivariate analysis
Bivariate analysis in descriptive statistics examines the relationship between two variables, extending univariate measures to assess joint variability and association. Covariance quantifies how two variables vary together, with the sample covariance given by the formula where and are observations, and are means, and is the sample size.[69] This measure can be positive, indicating that deviations from the mean occur in the same direction, negative for opposite directions, or zero for no linear association.[70] The Pearson correlation coefficient standardizes covariance to range between -1 and 1, providing a dimensionless measure of linear association strength and direction, defined as where and are standard deviations. Developed by Karl Pearson in his 1895 work on regression and inheritance, this coefficient assumes linearity and is widely used for continuous variables.[71] For instance, in anthropometric data, height and weight often show a moderate positive Pearson correlation, indicating that taller individuals tend to weigh more, though this varies by population and age.[72] For non-linear or ordinal associations, the Spearman rank correlation coefficient is applied, which computes the Pearson correlation on ranked data to capture monotonic relationships without assuming linearity.[73] Introduced by Charles Spearman in 1904, it is robust to outliers and non-normal distributions, making it suitable for ranked or non-parametric data.[74] Multivariate analysis generalizes these concepts to three or more variables, summarizing joint distributions and dependencies. For categorical variables, contingency tables display frequency distributions across multiple dimensions, enabling assessment of associations through observed frequencies and proportions.[75] A demographic example might cross-tabulate gender (rows), age group (columns), and education level (layers) in a three-way table, revealing patterns such as higher education rates among younger females, with cell frequencies normalized to marginal totals for comparison.[76] For high-dimensional continuous data, principal component analysis (PCA) reduces dimensionality by identifying orthogonal components that capture maximum variance, providing concise summaries of multivariate structure.[77] Developed by Harold Hotelling in 1933, PCA transforms original variables into uncorrelated principal components, ordered by explained variance, which aids in visualizing and interpreting complex datasets without loss of essential information.[78]References
- https://math.[arizona](/page/Arizona).edu/~jwatkins/505d/Lesson_5.pdf