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Sampling bias

Sampling bias in machine learning: [Fresh update]

Nov 17th, 2023

Machine learning has revolutionized the way data is processed and analyzed, making it a powerful tool for making predictions and decisions
across various domains. However, for machine learning models to be effective, they require high-quality data for training. One of the critical
challenges that machine learning practitioners face is sampling bias.
Sampling bias occurs when the data used to train a model is not representative of the population it aims to make predictions about. In this
article, we will explore the concept of sampling bias in machine learning and provide an updated perspective on this crucial issue.

Understanding Sampling Bias

Sampling bias arises when the process of selecting data for a machine learning model introduces systematic errors due to a non-random or nonrepresentative sample. These errors can lead to incorrect predictions and skewed results. It is essential to grasp the types and causes of sampling bias to address and mitigate this issue effectively.


Types of Sampling Bias

In the examination of alcohol consumption trends among adolescents, it becomes evident that the accuracy of our findings depends significantly on
the avoidance of various types of sampling bias. These biases can manifes  in different forms, as we explore in the following section, potentially distorting the true picture of adolescent alcohol use.

Selection Bias: Selection bias occurs when specific data points are systematically excluded or included in the dataset, resulting in a nonrepresentative sample. For example, in healthcare, if a study only includes healthy patients, the model trained on such data may not  generalize well to the general population.

Example: Selection bias is a common type of sampling bias. Let’s say we conduct a survey on alcohol consumption among adolescents and
deliberately exclude schools located in disadvantaged neighborhoods.

In this case, the sample of interviewed adolescents will not be representative of the entire population of adolescents. The findings
from this study may underestimate the actual issue of alcohol consumption among adolescents in disadvantaged neighborhoods.
Survivorship Bias: Survivorship bias happens when only the surviving or successful data points are included in the dataset, leading to
an overly optimistic view of the data. For example, analyzing only successful businesses without considering those that failed can result in
a biased model when predicting business success.

Example: Survivorship bias is another relevant example. Imagine if we only analyze adolescents who have survived episodes of excessive
alcohol consumption. Adolescents who have not experienced severe alcohol-related issues would not be included in our sample.

Consequently, our conclusions might give the impression that alcohol consumption among adolescents is less risky than it actually is.
Non-Response Bias: Non-response bias occurs when a portion of the data does not respond to a survey or data collection process. If this nonresponse is not random and follows a pattern, it can lead to a skewed dataset.

Example: Non-response bias occurs when some adolescents refuse to participate in a survey on alcohol consumption. If the adolescents who
choose not to participate have dif erent characteristics from those who do participate, the collected data can be biased. For instance, if the
most at-risk adolescents for alcohol consumption are less inclined to participate in the survey, the rates of alcohol consumption in the
sample may be underestimated.  


Volunteer Bias: Volunteer bias is introduced when participants or data points are self-selected. This can happen in surveys or voluntary data collection, leading to a dataset that represents a specific group of people with a particular interest.

Example: Volunteer bias occurs when adolescents voluntarily choose to participate in a survey on alcohol consumption. Volunteers may
dif er from the overall population of adolescents in terms of alcohol consumption behavior. If only adolescents concerned about their
alcohol consumption volunteer, the study’s findings may not be representative of the entire adolescent population.

Sampling Frame Bias: This type of bias occurs when the sampling frame used to collect data does not cover the entire population of
interest. For example, if a survey is conducted only in urban areas, it may not accurately represent the entire country’s demographics.

Example: Sampling frame bias is introduced when the sample does not encompass the entire target population. Let’s assume we conduct a
survey on alcohol consumption among adolescents but only collect data from schools. In this case, adolescents who have left school or are not
enrolled in it would not be represented. This can lead to bias, as the student population may dif er from the overall adolescent population
in terms of alcohol consumption.

Causes of Sampling Bias

Several factors can contribute to the emergence of sampling bias in machine learning:
Inadequate Data Collection: Incomplete or poorly designed data collection processes can lead to bias. For example, using convenience
sampling, where data is collected from easily accessible sources, can introduce significant bias.
Example: Imagine you are building a machine learning model to predict the popularity of music genres, and you collect data only from your friends and their music preferences. This convenience sampling introduces bias because it doesn’t represent a diverse population.
Data Preprocessing: Incorrect data preprocessing, such as data cleaning and data transformation, can inadvertently introduce bias.
Eliminating or imputing missing data without considering its implications can skew the dataset.


Data Imbalance: If the dataset is imbalanced, meaning one class or category is underrepresented, the model may struggle to make accurate
predictions for the underrepresented class.
Historical BiasHistorical biases present in the data used for training can perpetuate and reinforce stereotypes, leading to biased predictions.
Example: Consider a machine learning model that helps with hiring  decisions based on resumes. If historical hiring data has a bias towards
a certain gender or race, the model may perpetuate this bias by favoring candidates from the historically overrepresented group. 

Addressing Sampling Bias

To mitigate sampling bias in machine learning, several strategies and techniques can be employed:
Random Sampling: Ensure that the data collection process is as random as possible to minimize bias. Random sampling helps in obtaining a representative sample from the entire population. Stratified Sampling: Stratified sampling divides the dataset into subgroups or strata and then samples from each stratum. This helps ensure that each subgroup is adequately represented in the dataset. 


Data Augmentation: Augmenting the dataset by generating synthetic data for underrepresented classes can help balance class imbalances.
Bias Correction Techniques: Various algorithms and techniques are available to correct bias in the data, such as re-weighting samples or using fairness-aware machine learning models.


Diverse Data Sources: Incorporating data from diverse sources can help reduce bias by providing a more comprehensive view of the population.


Sampling bias in machine learning is a persistent challenge that can have far-reaching consequences, including reinforcing stereotypes, leading to
inaccurate predictions, and limiting the utility of machine learning models.
As the field of machine learning continues to advance, addressing sampling bias is of paramount importance. Employing proper data collection
methods, preprocessing techniques, and bias correction strategies can help mitigate this issue, ensuring that machine learning models make fair and
accurate predictions. Staying vigilant and informed about the latest developments in mitigating sampling bias is essential, as this article has provided a fresh update on this crucial issue in machine learning.