A Comprehensive Introduction to Evaluating Regression Models

Padhma M 30 Nov, 2023 • 14 min read


Machine learning models aim to understand patterns within data, enabling predictions, answers to questions, or a deeper understanding of concealed patterns. This iterative learning process involves the model acquiring patterns, testing against new data, adjusting parameters, and repeating until achieving satisfactory performance. The evaluation phase, essential for regression models, employs loss functions.

Role of Loss Functions in Model Evaluation

Loss functions compare the model’s predicted values with actual values, gauging its efficacy in mapping the relationship between X (feature) and Y (target). The loss, indicating the disparity between predicted and actual values, guides model refinement. A higher loss denotes poorer performance, demanding adjustments for optimal training.

Crucial Factors in Choosing Loss Functions

Selecting an appropriate loss function hinges on various factors such as the algorithm, data outliers, and the need for differentiability. With many options available, each with distinct properties, there is no universal solution. This article provides a comprehensive list of regression loss functions, outlining their advantages and drawbacks. Implementable across various libraries, the code examples use NumPy for enhanced transparency into the underlying mechanisms.

Let’s delve into the world of regression loss functions without delay.

This article was published as a part of the Data Science Blogathon

Loss function vs Cost function

The loss function is a function that calculates loss for one data point.

squared error
A loss function

The function that calculates loss for the entire data being used is called the cost function.

mse formula
A cost function

List of Top 13 Evaluation Metrics

Here is a list of 13 evaluation metrics

  • Mean Absolute Error (MAE)
  • Mean Bias Error (MBE)
  • Relative Absolute Error (RAE)
  • Mean Absolute Percentage Error (MAPE)
  • Mean Squared Error (MSE)
  • Root Mean Squared Error (RMSE)
  • Relative Squared Error (RSE)
  • Normalized Root Mean Squared Error (NRMSE)
  • Relative Root Mean Squared Error (RRMSE)
  • Root Mean Squared Logarithmic Error (RMSLE)
  • Hyber Loss
  • Log Cosh Loss
  • Quantile Loss

Mean Absolute Error (MAE)

Mean absolute error, or L1 loss, stands out as one of the simplest and easily comprehensible loss functions and evaluation metrics. It computes by averaging the absolute differences between predicted and actual values across the dataset. Mathematically, it represents the arithmetic mean of absolute errors, focusing solely on their magnitude, irrespective of direction. A lower MAE indicates superior model accuracy.

MAE formula is:

MAE evaluation metric formula


  • y_i = actual value
  • y_hat_i = predicted value
  • n = sample size

Python Code:


Pros of the MAE Evaluation Metric:

  • It is an easy to calculate evaluation metric.
  • All the errors are weighted on the same scale since absolute values are taken.
  • It is useful if the training data has outliers as MAE does not penalize high errors caused by outliers.
  • It provides an even measure of how well the model is performing.

Cons of the MAE evaluation metric:

  • Sometimes the large errors coming from the outliers end up being treated as the same as low errors.
  • MAE follows a scale-dependent accuracy measure where it uses the same scale as the data being measured. Hence it cannot be used to compare series’ using different measures.
  • One of the main disadvantages of MAE is that it is not differentiable at zero. Many optimization algorithms tend to use differentiation to find the optimum value for parameters in the evaluation metric.
  • It can be challenging to compute gradients in MAE.

Mean Bias Error (MBE)

In “Mean Bias Error,” bias reflects the tendency of a measurement process to overestimate or underestimate a parameter. It has a single direction, positive or negative. Positive bias implies an overestimated error, while negative bias implies an underestimated error. Mean Bias Error (MBE) calculates the mean difference between predicted and actual values, quantifying overall bias without considering absolute values. Similar to MAE, MBE differs in not taking the absolute value. Caution is needed with MBE, as positive and negative errors can cancel each other out.

The formula for MBE:

MBE evaluation metric formula
def mean_bias_error(true, pred):
    bias_error = true - pred
    mbe_loss = np.mean(np.sum(diff) / true.size)
    return mbe_loss

Pros of the MBE Evaluation Metric

  • MBE is a good measure if you want to check the direction of the model (i.e. whether there is a positive or negative bias) and rectify the model bias.

Cons of the MBE evaluation Metric

  • It is not a good measure in terms of magnitude as the errors tend to compensate each other.
  • It is not highly reliable because sometimes high individual errors produce low MBE.
  • As an evaluation metric, it can be consistently wrong in one direction. For example, if you’re trying to predict traffic patterns and it always shows lower traffic than what is actually observed.

Relative Absolute Error (RAE)

Relative Absolute Error is calculated by dividing the total absolute error by the absolute difference between the mean and the actual value. The formula for RAE is:

rae evaluation metric formula

where y_bar is the mean of the n actual values.

RAE measures the performance of a predictive model and is expressed in terms of a ratio. The value of RAE can range from zero to one. A good model will have values close to zero, with zero being the best value. This error shows how the mean residual relates to the mean deviation of the target function from its mean.

def relative_absolute_error(true, pred):
    true_mean = np.mean(true)
    squared_error_num = np.sum(np.abs(true - pred))
    squared_error_den = np.sum(np.abs(true - true_mean))
    rae_loss = squared_error_num / squared_error_den
    return rae_loss

Pros of the RAE Evaluation Metric

  • RAE can be used to compare models where errors are measured in different units.
  • In some cases, RAE is reliable as it offers protection from outliers.

Cons of the RAE evaluation Metric

  • One main drawback of RAE is that it can be undefined if the reference forecast is equal to the ground truth.

Mean Absolute Percentage Error (MAPE)

Calculate Mean Absolute Percentage Error (MAPE) by dividing the absolute difference between the actual and predicted values by the actual value. This absolute percentage is averaged across the dataset. MAPE, also known as Mean Absolute Percentage Deviation (MAPD), increases linearly with error. Lower MAPE values indicate better model performance.

mape evaluation metric formula
def mean_absolute_percentage_error(true, pred):
    abs_error = (np.abs(true - pred)) / true
    sum_abs_error = np.sum(abs_error)
    mape_loss = (sum_abs_error / true.size) * 100
    return mape_loss

Pros of the MAPE Evaluation Metric

  • MAPE is independent of the scale of the variables since its error estimates are in terms of percentage.
  • All errors are normalized on a common scale and it is easy to understand.
  • As MAPE uses absolute percentage errors, the problem of positive values and the negative values canceling each other out is avoided.

Cons of the MAPE evaluation Metric

  • MAPE faces a critical problem when the denominator becomes zero, resulting in a “division by zero” challenge.
  • MAPE exhibits bias by penalizing negative errors more than positive errors, potentially favoring methods with lower values.
  • Due to the division operation, MAPE’s sensitivity to alterations in actual values leads to varying loss for the same error. For example, an actual value of 100 and a predicted value of 75 result in a 25% loss, while an actual value of 50 and a predicted value of 75 yield a higher 50% loss, despite the identical error of 25.

Mean Squared Error (MSE)

MSE is one of the most common regression loss functions. In Mean Squared Error also known as L2 loss, we calculate the error by squaring the difference between the predicted value and actual value and averaging it across the dataset. MSE is also known as Quadratic loss as the penalty is not proportional to the error but to the square of the error. Squaring the error gives higher weight to the outliers, which results in a smooth gradient for small errors. Optimization algorithms benefit from this penalization for large errors as it is helpful in finding the optimum values for parameters. MSE will never be negative since the errors are squared. The value of the error ranges from zero to infinity. MSE increases exponentially with an increase in error. A good model will have an MSE value closer to zero.

mse evaluation metric formula
def mean_squared_error(true, pred):
    squared_error = np.square(true - pred) 
    sum_squared_error = np.sum(squared_error)
    mse_loss = sum_squared_error / true.size
    return mse_loss

Pros of the MSE Evaluation Metric

  • MSE values are expressed in quadratic equations. Hence when we plot it, we get a gradient descent with only one global minima.
  • For small errors, it converges to the minima efficiently. There are no local minima.
  • MSE penalizes the model for having huge errors by squaring them.
  • It is particularly helpful in weeding out outliers with large errors from the model by putting more weight on them.

Cons of the MSE evaluation Metric

  • One of the advantages of MSE becomes a disadvantage when there is a bad prediction. The sensitivity to outliers magnifies the high errors by squaring them.
  • MSE will have the same effect for a single large error as too many smaller errors. But mostly we will be looking for a model which performs well enough on an overall level.
  • MSE is scale-dependent as its scale depends on the scale of the data. This makes it highly undesirable for comparing different measures.
  • When a new outlier is introduced into the data, the model will try to take in the outlier. By doing so it will produce a different line of best fit which may cause the final results to be skewed.

Root Mean Squared Error (RMSE)

Root Mean Squared Error (RMSE) is a popular metric used in machine learning and statistics to measure the accuracy of a predictive model. It quantifies the differences between predicted values and actual values, squaring the errors, taking the mean, and then finding the square root. RMSE provides a clear understanding of the model’s performance, with lower values indicating better predictive accuracy.

It is computed by taking the square root of MSE. RMSE is also called the Root Mean Square Deviation. It measures the average magnitude of the errors and is concerned with the deviations from the actual value. RMSE value with zero indicates that the model has a perfect fit. The lower the RMSE, the better the model and its predictions. A higher RMSE indicates that there is a large deviation from the residual to the ground truth. RMSE can be used with different features as it helps in figuring out if the feature is improving the model’s prediction or not.

rmse evaluation metric formula
def root_mean_squared_error(true, pred):
    squared_error = np.square(true - pred) 
    sum_squared_error = np.sum(squared_error)
    rmse_loss = np.sqrt(sum_squared_error / true.size)
    return rmse_loss

Pros of the RMSE Evaluation Metric

  • RMSE is easy to understand.
  • It serves as a heuristic for training models.
  • It is computationally simple and easily differentiable which many optimization algorithms desire.
  • RMSE does not penalize the errors as much as MSE does due to the square root.

Cons of the RMSE Metric

  • Like MSE, RMSE is dependent on the scale of the data. It increases in magnitude if the scale of the error increases.
  • One major drawback of RMSE is its sensitivity to outliers and the outliers have to be removed for it to function properly.
  • RMSE increases with an increase in the size of the test sample. This is an issue when we calculate the results on different test samples.

Relative Squared Error (RSE)

In order to calculate Relative Squared Error, you take the Mean Squared Error (MSE) and divide it by the square of the difference between the actual and the mean of the data. In other words, we divide the MSE of our model by the MSE of a model which uses the mean as the predicted value.

rse evaluation metric formula
def relative_squared_error(true, pred):
    true_mean = np.mean(true)
    squared_error_num = np.sum(np.square(true - pred))
    squared_error_den = np.sum(np.square(true - true_mean))
    rse_loss = squared_error_num / squared_error_den
    return rse_loss

The output value of RSE is expressed in terms of ratio. It can range from zero to one. A good model should have a value close to zero while a model with a value greater than 1 is not reasonable.


Pros of the RSE Evaluation Metric

  • RSE is not scale-dependent. Hence it can be used to compare between models where errors are measured in different units.
  • RSE is not sensitive to the mean and the scale of predictions.

Normalized Root Mean Squared Error (NRMSE)

The Normalized RMSE is generally computed by dividing a scalar value. It can be in different ways like,

  • RMSE / maximum value in the series
  • RMSE / mean
  • RMSE / difference between the maximum and the minimum values (if mean is zero)
  • RMSE / standard deviation
  • RMSE / interquartile range

# implementation of NRMSE with standard deviation
def normalized_root_mean_squared_error(true, pred):
    squared_error = np.square((true - pred))
    sum_squared_error = np.sum(squared_error)
    rmse = np.sqrt(sum_squared_error / true.size)
    nrmse_loss = rmse/np.std(pred)
    return nrmse_loss


Opting for the interquartile range can be the most suitable choice, especially when dealing with outliers. NRMSE proves effective for comparing models with different dependent variables or when modifications like log transformation or standardization occur. This metric addresses scale-dependency issues, facilitating comparisons across models of varying scales or datasets.

Relative Root Mean Squared Error (RRMSE)

Relative Root Mean Squared Error (RRMSE) is a variant of Root Mean Squared Error (RMSE), gauging predictive model accuracy relative to the target variable range. It normalizes RMSE by the target variable range and presents it as a percentage for easy cross-dataset or cross-variable comparison. RRMSE, a dimensionless form of RMSE, scales residuals against actual values, allowing comparison of different measurement techniques.

  • Excellent when RRMSE < 10%
  • Good when RRMSE is between 10% and 20%
  • Fair when RRMSE is between 20% and 30%
  • Poor when RRMSE > 30%
rrmse evaluation metric formula
def relative_root_mean_squared_error(true, pred):
    num = np.sum(np.square(true - pred))
    den = np.sum(np.square(pred))
    squared_error = num/den
    rrmse_loss = np.sqrt(squared_error)
    return rrmse_loss


Root Mean Squared Logarithmic Error (RMSLE)

Root Mean Squared Logarithmic Error is calculated by applying log to the actual and the predicted values and then taking their differences. RMSLE is robust to outliers where the small and the large errors are treated evenly.

It penalizes the model more if the predicted value is less than the actual value while the model is less penalized if the predicted value is more than the actual value. It does not penalize high errors due to the log. Hence the model has a large penalty for underestimation than overestimation. This can be helpful in situations where we are not bothered by overestimation but underestimation is not acceptable.

rmsle evaluation metric formula
def root_mean_squared_log_error(true, pred):
    square_error = np.square((np.log(true + 1) - np.log(pred + 1)))
    mean_square_log_error = np.mean(square_error)
    rmsle_loss = np.sqrt(mean_square_log_error)
    return rmsle_loss


Pros of the RMSLE Evaluation Metric

  • RMSLE is not scale-dependent and is useful across a range of scales.
  • It is not affected by large outliers.
  • It considers only the relative error between the actual value and the predicted value.

Cons of the RMSLE evaluation Metric

  • It has a biased penalty where it penalizes underestimation more than overestimation.

Huber Loss

What if you want a function that learns about the outliers as well as ignores them? Well, Huber loss is the one for you. Huber loss is a combination of both linear and quadratic scoring methods. It has a hyperparameter delta (𝛿) which can be tuned according to the data. The loss will be linear (L1 loss) for values above delta and quadratic (L2 loss) for values below it. It balances and combines good properties of both MAE (Mean Absolute Error) and MSE (Mean Squared Error).

In other words, for loss values less than delta, MSE will be used and for loss values greater than delta, MAE will be used. The choice of delta (𝛿) is extremely critical because it defines our choice of the outlier. Huber loss reduces the weight we put on outliers for larger loss values by using MAE while for smaller loss values it maintains a quadratic function using MSE.

huber loss formula
def huber_loss(true, pred, delta):
    huber_mse = 0.5 * np.square(true - pred)
    huber_mae = delta * (np.abs(true - pred) - 0.5 * (np.square(delta)))
    return np.where(np.abs(true - pred) <= delta, huber_mse, huber_mae)
huber loss


Pros of the Huber Loss Evaluation Metric

  • It is differentiable at zero.
  • Outliers are handled properly due to the linearity above delta.
  • The hyperparameter, 𝛿 can be tuned to maximize model accuracy.

Cons of the Huber Loss evaluation Metric

  • The additional conditionals and comparisons make Huber loss computationally expensive for large datasets.
  • In order to maximize model accuracy, 𝛿 needs to be optimized and it is an iterative process.
  • It is differentiable only once.

Log Cosh Loss

Log cosh calculates the logarithm of the hyperbolic cosine of the error. This function is smoother than quadratic loss. It works like MSE but is not affected by large prediction errors. It is quite similar to Huber loss in the sense that it is a combination of both linear and quadratic scoring methods.

def log_cosh(true, pred):
    logcosh = np.log(np.cosh(pred - true))
    logcosh_loss = np.sum(logcosh)
    return logcosh_loss
log cosh loss


Pros of the Log Cosh Loss Evaluation Metric

  • It has the advantages of Huber loss while being twice differentiable everywhere. Some optimization algorithms like XGBoost favors double differentials over functions like Huber which can be differentiable only once.
  • It requires fewer computations than Huber.

Cons of the Log Cosh Loss evaluation Metric

  • It is less adaptive as it follows a fixed scale.
  • Compared to Huber loss, the derivation is more complex and requires much in-depth study.

Quantile Loss

Quantile regression loss function is applied to predict quantiles. The quantile is the value that determines how many values in the group fall below or above a certain limit. It estimates the conditional median or quantile of the response(dependent) variables across values of the predictor(independent) variables. The loss function is an extension of MAE except for the 50th percentile, where it is MAE. It provides prediction intervals even for residuals with non-constant variance and it does not assume a particular parametric distribution for the response.

quantile loss formula

𝛾 represents the required quantile. The quantiles values are selected based on how we want to weigh the positive and the negative errors.

In the loss function above, 𝛾 has a value between 0 and 1. When there is an underestimation, the first part of the formula will dominate and for overestimation, the second part will dominate. The chosen value of quantile(𝛾) gives different penalties for over-prediction and under prediction. When 𝛾 = 0.5, underestimation and overestimation are penalized by the same factor and the median is obtained. When the value of 𝛾 is larger, overestimation is penalized more than underestimation. For example, when 𝛾 = 0.75 the model will penalize overestimation and it will cost three times as much as underestimation. Optimization algorithms based on gradient descent learn from the quantiles instead of the mean.

def quantile_loss(true, pred, gamma):
    val1 = gamma * np.abs(true - pred)
    val2 = (1-gamma) * np.abs(true - pred)
    q_loss = np.where(true >= pred, val1, val2)
    return q_loss
quantile loss


Pros of the Quantile Loss Evaluation Metric

  • It is particularly useful when we are predicting an interval instead of point estimates.
  • This function can also be used to calculate prediction intervals in neural nets and tree-based models.
  • It is robust to outliers.

Cons of the Quantile Loss evaluation Metric

  • Quantile loss is computationally intensive.
  • If we use a squared loss to measure the efficiency or if we are to estimate the mean, then quantile loss will be worse.


This comprehensive guide navigated through diverse regression loss functions, shedding light on their applications, advantages, and drawbacks. The article demystified complex metrics like MAE, MBE, RAE, MAPE, MSE, RMSE, RSE, NRMSE, RRMSE, RMSLE, and introduced specialized losses like Huber, Log Cosh, and Quantile. It emphasized the nuanced factors influencing loss function selection, from algorithm types to outlier handling.

Thank you for reading all the way down here! I hope this article was helpful in your learning journey. I would love to hear in the comments about any other loss functions that I have missed. Happy Evaluating!

Frequently Asked Questions

Q1. What is the formula for the root relative squared error?

A. RRSE = sqrt(SSR/SST) * 100%

SSR: Sum of squared residuals
SST: Total sum of squares
RRSE assesses regression model accuracy, presented as a percentage of the dependent variable range. Lower RRSE signifies better predictive accuracy.

Q2. What is the difference between RMSE and relative RMSE?

A. RMSE measures average predicted vs. actual variable differences. Relative RMSE normalizes this by the actual values range, expressed as a percentage. RMSE uses variable units, while relative RMSE is a percentage.

Q3. What is an acceptable MAPE for forecasting?

A. No universal MAPE standard; varies by industry, forecast horizon, and more. Generally, a MAPE under 5% is good, 5-10% is acceptable, and over 10% is poor accuracy.

Q4. How can I improve my MAPE?

A. High-Quality Data: Utilize clean, accurate, and representative data for model training.
Suitable Model: Select a model fitting your data and forecasting horizon.
Optimize Model: Adjust parameters or include additional features for model optimization.
Performance Monitoring: Regularly monitor model performance and make necessary adjustments.


Padhma M 30 Nov 2023

Frequently Asked Questions

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Responses From Readers


Ron 29 May, 2022

This is an excellent article. I feel it was very well laid out, structured, and easy to understand. One questions on the Python formula for Relative Root Mean Squared Error (RRMSE), is it missing the division by n?

Chiamaka 29 Jun, 2022

I read through to the end and it was very educative. Thank you. Question: If you were to choose an evaluator for the comparison of predictions from multiple linear and nonlinear models trained using the same data with small outliers, which top three evaluators will you choose and why?

Sungil 14 Feb, 2023

Thanks for the useful resource. Just want to let you know that you missed 1/n part in the Root Mean Squared Logarithmic Error (RMSLE).

Dennis 04 Oct, 2023

In the definition of RRMSE, if the y_i = actual value, are all zero, the RRMSE is square root of 1/n, not 1 Why is the denominator summed, but the numerator averaged? Also, should the denominator include y_i, the actual value, instead of y_hat_i , predicted value?

Eric 14 Dec, 2023

Shouldn't the denominator in th. relative and normalized rmse be a function of the true values and not predicted values? It looks like the expressions and the python code examples all make the denominators a function of the predicted values.

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