A/B Testing
A/B testing (also called split testing, bucket testing, or an online controlled experiment) is a randomized controlled experiment that compares two variants, a control (A) and a treatment (B), by randomly…
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A/B testing (also called split testing, bucket testing, or an online controlled experiment) is a randomized controlled experiment that compares two variants, a control (A) and a treatment (B), by randomly…
ARIMA (Autoregressive Integrated Moving Average) is a class of statistical models for analyzing and forecasting time series data, specified by three non-negative integer orders written as ARIMA(p, d, q): p is…
AUC (Area Under the Curve), most often the area under the ROC curve (AUC-ROC), is a threshold-independent evaluation metric that measures how well a binary classification model ranks positive cases above…
Area under the curve (AUC) is a single scalar metric that summarizes the performance of a binary classifier or diagnostic test across all possible decision thresholds by integrating the area beneath a…
Bayes' theorem (also called Bayes' rule or Bayes' law) is a fundamental theorem of probability theory that describes how to update the probability of a hypothesis given new evidence.
Bayesian inference is a method of statistical inference in which Bayes' theorem is used to update the probability of a hypothesis as new evidence or data becomes available.
A Bayesian network (also called a belief network, Bayes net, directed graphical model, or probabilistic causal network) is a probabilistic graphical model that represents a set of random variables and their…
Bayesian statistics is a statistical paradigm in which probability expresses a degree of belief that is updated as evidence arrives, using Bayes' theorem.
The bias-variance tradeoff is a foundational concept in machine learning and statistics that describes the tension between two competing sources of error in predictive models: bias (error from overly…
Calibration in machine learning is the property that the probability scores produced by a probabilistic classifier match the empirical frequency of the predicted event: a model that assigns a confidence of 0.8…
Categorical data, also called qualitative data, is data whose values are discrete labels or groups (such as colors, country names, or blood types) rather than measurable quantities, so they cannot be…
Causal inference is the field of study concerned with drawing conclusions about cause-and-effect relationships from data, answering questions of the form "what would happen to outcome Y if we intervened on…
Concept drift is the change over time in the statistical relationship between a model's inputs and its target, formally when the joint distribution P(X, Y) (and in the most damaging case the conditional P(Y |…
A conditional random field (CRF) is a discriminative probabilistic model for structured prediction, used most often to assign a label to every position in an input sequence.
A continuous feature is a numeric input variable in machine learning and statistics that can take any value within a range, including decimals and fractions, rather than a fixed set of categories or counts.
Convenience sampling (also called grab sampling, accidental sampling, or opportunity sampling) is a non-probability sampling method in which data points or participants are selected because they are easy to…
Counterfactual fairness is a formal definition of algorithmic fairness rooted in causal inference: a prediction is counterfactually fair toward an individual if it would remain unchanged in a counterfactual…
The curse of dimensionality is the set of problems that arise when data has a large number of features (dimensions): as dimensions increase, the volume of the space grows exponentially, the available data…
Data analysis is the process of inspecting, cleaning, transforming, and modeling data to discover useful information, draw conclusions, and support decision-making.
Data science is an interdisciplinary field that uses statistics, programming, and domain expertise to extract knowledge and insights from structured and unstructured data.
Differential privacy is a mathematical definition of privacy that guarantees the output of an analysis is essentially unchanged whether or not any single individual's record is included in the input
Distributionally robust optimization (DRO) is a framework for making decisions when the probability distribution of uncertain inputs is not known exactly.
Earth Mover's Distance (EMD), also known as the Wasserstein-1 distance, Kantorovich-Rubinstein metric, or Mallows's distance
The Elo rating system, as applied to AI models, is a method for turning a pile of head-to-head preference votes into a single number per model, so that large language models and chatbots can be ranked on a…
An estimator is a rule, function, or algorithm that takes observed data and produces a value intended to approximate some unknown quantity, typically a parameter of a probability distribution or a function…
The Expectation-Maximization (EM) algorithm is an iterative method for finding maximum likelihood or maximum a posteriori (MAP) estimates of the parameters of statistical models that involve latent…
Expected calibration error (ECE) is a metric that measures how well a classifier's predicted confidence matches its observed accuracy.
Experimenter's bias (also called the observer-expectancy effect, experimenter expectancy effect, or experimenter effect) is a type of cognitive bias in which a researcher's expectations or beliefs about the…
The F1 score (also written as F1-score, F-score, or F-measure) is the harmonic mean of precision and recall, calculated as $$F_1 = \frac{2 \cdot (\text{Precision} \cdot \text{Recall})}{\text{Precision} +…
The false negative rate (FNR), also known as the miss rate, is the proportion of actual positive instances that a model or test incorrectly classifies as negative, computed as FNR = FN / (FN + TP).
The false positive rate (FPR) is the proportion of actual negative cases that a test, model, or decision process incorrectly classifies as positive, defined as FPR = FP / (FP + TN) where FP is the number of…
A false negative (FN), also called a Type II error or a miss, is an instance whose true label is positive but that a classification model or test predicts as negative: a real positive case that the model fails…
A false positive (FP), also called a Type I error or a false alarm, is an instance whose true label is negative but whose predicted label is positive: the classifier raises an alarm where none was warranted.
A Gaussian process (GP) is a probabilistic machine learning model defined as a collection of random variables, any finite number of which have a joint Gaussian distribution.
A generalized linear model (GLM) is a flexible extension of ordinary linear regression that allows the response variable to follow any distribution from the exponential family, not just the normal distribution
Goodhart's law states that "when a measure becomes a target, it ceases to be a good measure": any statistical regularity or metric tends to break down once it is used as a target for control or decision-making.
Importance sampling (often abbreviated IS) is a Monte Carlo method for estimating the expectation of a function under a target probability distribution $$p$$ by drawing samples from a different proposal…
Independently and identically distributed (abbreviated i.i.d., iid, or IID) data is a collection of random variables in which each variable has the same probability distribution as the others and all variables…
Information theory is the mathematical study of the quantification, storage, and communication of information, founded by Claude Shannon in his 1948 paper "A Mathematical Theory of Communication." Shannon…
Inter-rater agreement is the degree of consensus among two or more independent raters when they label or score the same set of items.
The Iris dataset, sometimes referred to as Fisher's Iris dataset or the Iris flower dataset, is a multivariate dataset introduced by the British statistician and biologist Ronald Fisher in his 1936 paper "The…
Kullback-Leibler divergence, often abbreviated KL divergence and written $$D_{\mathrm{KL}}(P \parallel Q)$$
The Kalman filter is a recursive algorithm that estimates the hidden state of a dynamic system from a sequence of noisy measurements.
L1 loss is a regression loss function equal to the average of the absolute differences between predicted values and target values, written as $$\frac{1}{n} \sum \lvert y_i - \hat{y}_i \rvert$$.
L2 loss is the squared-error loss function: for a true value $$y$$ and a predicted value $$\hat{y}$$, it is the squared difference $$(y - \hat{y})^2$$, and averaging it across a dataset gives the mean squared…
Latent Dirichlet allocation (LDA) is a generative probabilistic model that discovers the hidden thematic structure in a collection of documents by treating each document as a mixture of a small number of…
Least squares regression is a statistical method that fits a model to data by choosing the parameters that minimize the sum of the squared residuals, the squared differences between the observed values and the…
Leo Breiman (January 27, 1928 to July 5, 2005) was an American statistician at UC Berkeley whose work on tree-based prediction supplied machine learning with several of its most durable tools.
Linear Discriminant Analysis (LDA) is a classical statistical method for classification and dimensionality reduction that finds the linear combination of features which best separates two or more classes by…
Linear regression is a statistical method that models the relationship between one or more independent variables (the predictors or features) and a continuous dependent variable (the response) by fitting a…
A linear model is any statistics or machine learning model whose prediction is a linear function of its input features, of the form f(x) = g(w_1 x_1 + w_2 x_2 + ... + w_p x_p + b)
Log-odds, also known as the logit, is a mathematical transformation that converts a probability value between 0 and 1 into a real number spanning from negative infinity to positive infinity.
Logistic regression is a statistical model for a binary response. It represents the conditional probability of one outcome as the logistic transformation of a linear predictor.
In machine learning and statistics, logits refer to the raw, unnormalized scores output by a model before they are converted into probabilities.
Markov Chain Monte Carlo (MCMC) is a class of algorithms for drawing samples from a probability distribution by constructing a Markov chain whose stationary distribution equals the target distribution .
A Markov chain is a stochastic process in which the probability of the next state depends only on the current state and not on the sequence of states that came before it.
The Markov property is the condition that a stochastic process is memoryless: the future of the process depends only on its present state, not on the path the process took to reach that state.
Maximum likelihood estimation (MLE) is the method of choosing the parameters of a probability model so that they make the observed data as probable as possible: given a parametric model with density (or mass)…
Mean Absolute Error (MAE) is a regression accuracy metric and loss function that measures the average absolute difference between predicted values and actual observed values
Mean Squared Error (MSE), also called mean squared deviation (MSD), is the average of the squared differences between predicted values and actual (observed) values