Cited by 7,178Meteorological Phenomena and SimulationsHydrology and Watershed Management StudiesForecasting Techniques and Applications

Root mean square error (RMSE) or mean absolute error (MAE)? – Arguments against avoiding RMSE in the literature

Open access
Chai, T., Draxler, R. R.·2014·Geoscientific Model Development

Published version of a preprint

Abstract

Abstract. Both the root mean square error (RMSE) and the mean absolute error (MAE) are regularly employed in model evaluation studies. Willmott and Matsuura (2005) have suggested that the RMSE is not a good indicator of average model performance and might be a misleading indicator of average error, and thus the MAE would be a better metric for that purpose. While some concerns over using RMSE raised by Willmott and Matsuura (2005) and Willmott et al. (2009) are valid, the proposed avoidance of RMSE in favor of MAE is not the solution. Citing the aforementioned papers, many researchers chose MAE over RMSE to present their model evaluation statistics when presenting or adding the RMSE measures could be more beneficial. In this technical note, we demonstrate that the RMSE is not ambiguous in its meaning, contrary to what was claimed by Willmott et al. (2009). The RMSE is more appropriate to represent model performance than the MAE when the error distribution is expected to be Gaussian. In addition, we show that the RMSE satisfies the triangle inequality requirement for a distance metric, whereas Willmott et al. (2009) indicated that the sums-of-squares-based statistics do not satisfy this rule. In the end, we discussed some circumstances where using the RMSE will be more beneficial. However, we do not contend that the RMSE is superior over the MAE. Instead, a combination of metrics, including but certainly not limited to RMSEs and MAEs, are often required to assess model performance.

AI snapshot

Written from the paper’s full text.

TL;DR

This technical note argues against the recommendation by Willmott and Matsuura (2005) and Willmott et al. (2009) to abandon RMSE in favor of MAE, showing that RMSE is not ambiguous in meaning and is more appropriate when errors are expected to be Gaussian. The authors also demonstrate that RMSE satisfies the triangle inequality required of a distance metric, contrary to the earlier claims, while concluding that a combination of metrics is often needed.

Key findings

  • RMSE is not ambiguous in its meaning, contrary to Willmott et al. (2009); it is more appropriate than MAE when the error distribution is expected to be Gaussian, whereas MAE suits uniformly distributed errors.
  • Using randomly generated zero-mean, unit-variance Gaussian pseudo-errors, RMSEs converge to the true standard deviation (SE = 1) once n ≥ 100, with standard deviation within 5% of truth; MAEs converge to 0.8, approximating the expectation of √(2/π). Neither metric is robust at n = 4 or n = 10.
  • RMSE satisfies the triangle inequality required for a distance metric: the example in Willmott et al. (2009) (d(a,c)=4, d(a,b)=2, d(b,c)=3) actually represents the MSE, not the RMSE, and the L2-norm proof in Eqs. (5)–(7) shows RMSE does satisfy it.
  • RMSE has practical advantages: it avoids absolute values (useful for gradients/sensitivities and data assimilation cost functions) and is generally better at discriminating among model results by weighting unfavorable conditions more heavily.

Methods

  • Technical note / commentary design: no new observational dataset; the argument is analytical, using norm theory (L1- and L2-norms linked to MAE and RMSE) and a proof of the triangle inequality for the L2-norm.
  • A Monte Carlo-style demonstration: five sets of pseudo-errors of size n = 4, 10, 100, 1000, 10 000, and 1000 000 drawn from a zero-mean, unit-variance Gaussian distribution with different random seeds, comparing RMSEs and MAEs (Table 1).
  • Reference to prior empirical example: Chai et al. (2009) reported RMSE-to-MAE ratios of 1.63 to 2.29 for model NO2 column predictions versus SCIAMACHY satellite observations.

Limitations

  • Stated by the authors: the analysis assumes unbiased errors and does not consider uncertainties from observation errors or the method used to compare model and observations; all statistics are less useful with limited sample sizes (e.g., n = 4 or 10), where presenting the errors themselves may be more appropriate; for non-normal distributions, additional moments (mean, variance, skewness, flatness) are needed; outliers several orders larger than other samples may justifiably be removed when sample numbers are limited, and systematic errors may need removal before computing RMSEs.
  • Evident from the text: the demonstration relies on synthetic Gaussian pseudo-errors rather than real model–observation comparisons, so the convergence results are conditional on the Gaussian assumption; the authors explicitly do not claim RMSE is superior to MAE, so the note offers guidance rather than a definitive metric selection rule.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.5194/gmd-7-1247-2014

Cites

13 of 13 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

4 of 8 cited sources checked · 4 supported · 0 flagged

  • Premisecitation

    Willmott and Matsuura (2005) demonstrated that while keeping the MAE constant at 2.0, the RMSE varies from 2.0 to 4.0.

    “Using hypothetical sets of four errors, Willmott and Matsuura (2005) demonstrated that while keeping the MAE as a constant of 2.0, the RMSE varies from 2.0 to 4.0.”
    Quote in the paperSubstantial claim1 of 1 sources checked
    • cites Willmott and Matsuura (2005) source
  • Findingreasoning

    The RMSE satisfies the triangle inequality requirement for a distance metric.

    “In addition, we show that the RMSE satisfies the triangle inequality requirement for a distance metric, whereas Willmott et al. (2009) indicated that the sums-ofsquares-based statistics do not satisfy this rule.”
    • Also stated in: § 3 Triangle inequality of a metric · boosted
      “This proves that RMSE satisfies the triangle inequality required for a distance function metric.”
    • Also stated in: § 4 Summary and discussion · boosted
      “In addition, we demonstrate that the RMSE satisfies the triangle inequality required for a distance function metric.”
    Quote in the paperSubstantial claim
  • Findingown data

    When the sample size reaches 100 or above, using calculated RMSEs one can re-construct the error distribution close to its truth with its standard deviation within 5% of its truth (SE = 1).

    “When the sample size reaches 100 or above, using the calculated RMSEs one can re-construct the error distribution close to its “truth” or “exact solution”, with its standard deviation within 5 % to its truth (i.e., SE = 1).”
    Quote in the paperSubstantial claim
    • The authors state this as an assumption: The errors are unbiased and follow a normal distribution when presenting the RMSE.
    • The authors state this as an assumption: The uncertainties brought in by observation errors or the method used to compare model and observations are not considered.
  • Findingown data

    Neither the RMSEs nor the MAEs are robust when only 4 or 10 samples are used to calculate those values.

    “For instance, Table 1 shows that neither the RMSEs nor the MAEs are robust when only 4 or 10 samples are used to calculate those values.”
    Quote in the paperNot a substantial claim
    • The authors state this as an assumption: The errors are unbiased and follow a normal distribution when presenting the RMSE.
    • The authors state this as an assumption: The uncertainties brought in by observation errors or the method used to compare model and observations are not considered.
  • Interpretationreasoning

    The RMSE is more appropriate to represent model performance than the MAE when the error distribution is expected to be Gaussian.

    “The RMSE is more appropriate to represent model performance than the MAE when the error distribution is expected to be Gaussian.”
    • Also stated in: § 1 Introduction · plain
      “When the error distribution is expected to be Gaussian and there are enough samples, the RMSE has an advantage over the MAE to illustrate the error distribution.”
    • Also stated in: § 4 Summary and discussion · plain
      “We present that the RMSE is not ambiguous in its meaning, and it is more appropriate to use than the MAE when model errors follow a normal distribution.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: Model errors are likely to have a normal distribution rather than a uniform distribution.
  • Classified as probably not a substantial claim. 10
  • May combine two claims. 2
  • The quote only partly states this claim. 1
  • Classified as probably not a substantial claim.

    Premisecitation

    The RMSE tends to become increasingly larger than the MAE as the distribution of error magnitudes becomes more variable.

    “They summarized that the RMSE tends to become increasingly larger than the MAE (but not necessarily in a monotonic fashion) as the distribution of error magnitudes becomes more variable.”
    Quote in the paperSubstantial claim1 of 1 sources checked
    • cites Willmott and Matsuura (2005) source
  • Classified as probably not a substantial claim. May combine two claims.

    Premisecitation

    The RMSE varies with the variability of the error magnitudes, the total-error or average-error magnitude (MAE), and the sample size n.

    “They concluded that the RMSE varies with the variability of the the error magnitudes and the total-error or averageerror magnitude (MAE), and the sample size n.”
    Quote in the paperSubstantial claim1 of 1 sources checked
    • cites Willmott and Matsuura (2005) source
  • May combine two claims.

    Premisecitation

    Willmott and Matsuura (2005) proved that the RMSE is not equivalent to the MAE, and one cannot easily derive the MAE value from the RMSE (and vice versa).

    “Willmott and Matsuura (2005) have simply proved that the RMSE is not equivalent to the MAE, and one cannot easily derive the MAE value from the RMSE (and vice versa).”
    Quote in the paperNot a substantial claim1 of 1 sources checked
    • cites Willmott and Matsuura (2005) source
  • The quote only partly states this claim.

    Findingown data

    The MAEs converge to 0.8, an approximation to the expectation of sqrt(2/pi).

    “Table 1 shows that the MAEs converge to 0.8, an approximation to the expectation of 2 π .”
    Quote in the paperNot a substantial claim
    • The authors state this as an assumption: The errors are unbiased and follow a normal distribution when presenting the RMSE.
    • The authors state this as an assumption: The uncertainties brought in by observation errors or the method used to compare model and observations are not considered.
  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Premisenone given

    When both metrics are calculated, the RMSE is by definition never smaller than the MAE.

    “When both metrics are calculated, the RMSE is by definition never smaller than the MAE.”
    Quote in the paperNot a substantial claim
  • Probably not a substantial claim: may not pass S4 (based).

    Premisenone given

    The MAE is suitable to describe uniformly distributed errors.

    “The MAE is suitable to describe uniformly distributed errors.”
    Quote in the paperNot a substantial claim
  • Probably not a substantial claim: may not pass S2 (checkable), S3 (load-bearing).

    Interpretationreasoning

    For other kinds of distributions, more statistical moments of model errors, such as mean, variance, skewness, and flatness, are needed to provide a complete picture of the model error variation.

    “For other kinds of distributions, more statistical moments of model errors, such as mean, variance, skewness, and flatness, are needed to provide a complete picture of the model error variation.”
    Quote in the paperNot a substantial claim
  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    The ratio of RMSE to MAE ranged from 1.63 to 2.29 in Chai et al. (2009) model NO2 column predictions compared to SCIAMACHY satellite observations.

    “The ratio of RMSE to MAE ranged from 1.63 to 2.29 (see Table 1 of Chai et al., 2009).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Chai et al., 2009 source
  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Interpretationnone given

    In many model sensitivity studies that use only RMSE, a detailed interpretation is not critical because variations of the same model will have similar error distributions.

    “In many of the model sensitivity studies that use only RMSE, a detailed interpretation is not critical because variations of the same model will have similar error distributions.”
    Quote in the paperNot a substantial claim
  • Probably not a substantial claim: may not pass S2 (checkable).

    Premisereasoning

    Any single metric provides only one projection of the model errors and emphasizes a certain aspect of the error characteristics.

    “As every statistical measure condenses a large number of data into a single value, it only provides one projection of the model errors emphasizing a certain aspect of the error characteristics of the model performance.”
    • Also stated in: § 4 Summary and discussion · plain
      “As stated earlier, any single metric provides only one projection of the model errors and, therefore, only emphasizes a certain aspect of the error characteristics.”
    Quote in the paperNot a substantial claim
  • Probably not a substantial claim: may not pass S2 (checkable).

    Interpretationreasoning

    A combination of metrics, including but not limited to RMSEs and MAEs, are often required to assess model performance.

    “A combination of metrics, including but certainly not limited to RMSEs and MAEs, are often required to assess model performance.”
    • Also stated in: § 4 Summary and discussion · plain
      “A combination of metrics, including but certainly not limited to RMSEs and MAEs, are often required to assess model performance.”
    Quote in the paperNot a substantial claim
  • Classified as probably not a substantial claim.

    Premisenone given

    The RMSE penalizes variance as it gives errors with larger absolute values more weight than errors with smaller absolute values.

    “While the MAE gives the same weight to all errors, the RMSE penalizes variance as it gives errors with larger absolute values more weight than errors with smaller absolute values.”
    Quote in the paperSubstantial claim
12 of 12
  • The authors state this as an assumption.

    The errors are unbiased and follow a normal distribution when presenting the RMSE.

    “The underlying assumption when presenting the RMSE is that the errors are unbiased and follow a normal distribution.”
    “We also assume the error sample set is unbiased.”
  • The paper relies on this without support.

    Model errors are likely to have a normal distribution rather than a uniform distribution.

    “Because model errors are likely to have a normal distribution rather than a uniform distribution, the RMSE is a better metric to present than the MAE for such a type of data.”
  • The authors state this as an assumption.

    The uncertainties brought in by observation errors or the method used to compare model and observations are not considered.

    “The uncertainties brought in by observation errors or the method used to compare model and observations are not considered here.”
  • The paper relies on this without support.

    There are often hundreds of observations available to calculate model statistics.

    “Fortunately, there are often hundreds of observations available to calculate model statistics, unlike the examples with n = 4 (Willmott and Matsuura, 2005) and n = 10 ( Willmott et al., 2009).”
4 of 4

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

Each contradiction between two passages listed here was confirmed by an independent check.

  • Table reports an MAE larger than its RMSE

    Minor

    For the fourth set of four errors, the table lists RMSE = 1.02 and MAE = 1.16. This contradicts the paper's statement that RMSE is never smaller than MAE when both are calculated for the same errors.

1 of 1

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Authors
Chai, T., & Draxler, R. R.
Affiliations
National Oceanic and Atmospheric Administration (US)
Venue
Geoscientific Model Development
Published
2014-06-30
Volume
7
Issue
3
Pages
1247-1250
ISSN
  • 1991-959X (ISSN-L)
  • 1991-9603
OpenAlex
W2102148524
Cited by
7,178
References
12
Citations by year
2026: 1015 · 2025: 1120 · 2024: 980 · 2023: 873 · 2022: 751 · 2021: 683 · 2020: 538 · 2019: 411 · 2018: 324 · 2017: 235 · 2016: 175 · 2015: 59 · 2014: 13
FWCI
106.31
Type
journal article
Language
en
Open access
Locations
  • Geoscientific model development (publishedVersion, cc-by)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
Funders
National Oceanic and Atmospheric Administration (NA09NES4400006)
Topics
  • Physical Sciences › Earth and Planetary Sciences › Atmospheric Science › Meteorological Phenomena and Simulations
  • Physical Sciences › Environmental Science › Water Science and Technology › Hydrology and Watershed Management Studies
  • Social Sciences › Decision Sciences › Management Science and Operations Research › Forecasting Techniques and Applications
Keywords
root mean square error, mean absolute error, model evaluation metrics, error distribution, Gaussian errors, triangle inequality, distance measures, model evaluation, hydrological model evaluation

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