Photograph of no mold spores, one mite fecal.Definition of Type 1 Errors & Type 2 Errors
Effects of Type I or Type II Errors on Mold or Environmental Testing

  • ENVIRONMENTAL TEST ERROR TYPES - CONTENTS: what are the two types of bad mistakes that are often made in building mold or other environmental tests & inspections. Borrowing from math and statistics we describe type 1 and type 2 errors as they apply to the building environment.
  • POST a QUESTION or READ FAQs about how to report the density or level of mold or other particles found on indoor surfaces or in indoor dust samples
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Definition & examples of Type 1 & Type 2 errors:

Classes of testing or statistical errors applied to mold or other environmental tests, inspections & reports can be divided into two basic mistakes or error types as we explain here. We include the definitions of type 1 and type 2 errors and we give examples of type 1 and 2 errors in building inspection and testing for mold contamination.

It is important to understand these types of environmental testing mistakes in order to reduce unnecessary risks to building occupants (failing to detect and report an environmental hazard) as well as to avoid wasting people's time and money focusing on on-problems (reporting as hazards conditions that are in fact not hazardous).

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Definition & Practical Examples of Type I and Type II Errors in Building Environmental Testing for Mold or Other Contaminants

Photograph of Aspergillus niger spores.

Types of Reporting Errors in Buildings: definitions of Type 1 Errors & Type 2 Errors

Using building environmental testing for mold contamination as an example this article describes the types of errors that may be made by thinking, technical, or procedural errors during an investigation or test.

Type One Errors - missing a problem that's present: Occasional occurrences of certain mold genera in samples might suggest a hidden or un-noticed mold problem in the building somewhere other than at the spot from which the sample was collected. This is particularly true if the sample was collected by someone who is not expert at building science, indoor air quality, mycology, and related disciplines.

Examples of Type I errors during a mold investigation include:

Type II Errors - asserting that a problem is present when it is not: Conversely, occasional occurrences of certain mold in samples might also seem to point a problem in a building where in fact none is present.

Photograph: typical cosmetic bluestain mold on new framing lumber, floor joists - © Daniel Friedman

This is a greater risk where mold "counts" are used in air sampling than it it is when surface sampling is combined with visual inspection. Occupant indoor air or environment-related complaints or a building history of leaks can suggest that additional investigation is in order.

Examples of Type II errors during a building mold investigation include:

See MOLD / ENVIRONMENTAL EXPERT, HIRE ? for help in determining when the risk indicators justify the cost and trouble of hiring a mold or environmental expert

To avoid both Type I and Type II errors in measuring toxic or allergenic mold exposure the building consultant needs to understand mycology (e.g. what mold is likely to grow in buildings), the significance of the particles found (e.g. Pen/Asp spore chains vs. individual spores), the history, construction, and materials in the building and the details of the inspection itself when interpreting the importance of low levels of mold in building samples.

The articles listed below provide more examples of sources of Type 1 or Type 2 errors during building mold or environmental inspection, testing, lab sample analysis, & reporting.

Environmental Testing Articles


Continue reading at MOLD TESTING & SAMPLING MISTAKES for more examples of how mold testing goes wrong, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



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