Overview
ISO 3443-2:1979 - "Tolerances for building - Part 2: Statistical basis for predicting fit between components having a normal distribution of sizes" provides the statistical foundation for assessing dimensional variability in building construction. The standard addresses how normally (Gaussian) distributed component sizes combine to determine joint widths and the probability of satisfactory fit or misfit in assemblies. It applies where component variability is predictable and approximately Gaussian.
Key topics
- Normal (Gaussian) distribution: Defines the use of the mean and standard deviation to describe size distributions and shows the familiar probability limits (≈68.27%, 95.45%, 99.73% for ±1, ±2, ±3σ).
- Combination of independent random variables: Uses the root-sum-square rule - the standard deviation of combined variability equals the square root of the sum of the squares of individual standard deviations - to predict total dimensional variability in assemblies.
- Fit and joint width prediction: Relates predicted dimensional variability to allowable joint width limits (clearance or joint clearance) and quantifies the probability of exceeding minimum or maximum joint limits (misfit).
- Probability of failure and confidence limits: Explains how to compute probabilities that joint limits will be infringed and describes estimating population parameters from samples with confidence intervals (e.g., 95% confidence).
- Sampling and calculation (Annex): Provides practical methods for random sampling, computing arithmetic mean and standard deviation from observations, and guidance on representativeness of small site samples (includes a worked example).
Applications
- Predicting joint clearances and the risk of misfit when assembling prefabricated elements or site-built components.
- Selecting appropriate work sizes and specifying tolerances so that jointing techniques can perform satisfactorily.
- Designing quality-control and inspection procedures (sample sizes, confidence limits) for tolerances in building manufacture and erection.
- Evaluating compatibility between components from different suppliers and ensuring functional performance of joints.
Who uses this standard
- Structural and façade engineers, architects, and detailing specialists
- Manufacturers of prefabricated building components
- Construction quality managers, site engineers and contractors
- Tolerancing analysts and standards committees setting acceptance criteria
Related standards
- ISO 3207 - Statistical interpretation of data (confidence limits, sample sizing)
- ISO 3443-1 and ISO 4464 - Other parts of the series covering principles, vocabulary, identification and relationship of tolerances
ISO 3443-2:1979 is essential when tolerancing decisions must be grounded in statistical reality-helping practitioners translate measured variability into reliable predictions of fit, joint performance, and acceptable risk.