machine learning - Overfitting and Underfitting - Cross Validated

I have made some research about overfitting and underfitting, and I have understood what they exactly are, but I cannot find the reasons. What are the main reasons for overfitting and underfitting

Overfitting DataRobot AI Wiki

Overfitting, Underfitting, Cross-Validation, and the Bias-Variance Tradeoff - Justin Skycak

An Underfitting and Overfitting in Machine Learning

Overcoming overfitting a model in Machine Learning, by Abhishek Shah

Overfitting and underfitting in machine learning

Underfitting vs. Overfitting — scikit-learn 1.4.1 documentation

machine learning - Underfitting? Validation scores above training scores - Cross Validated

miro.medium.com/v2/resize:fit:1036/1*Hgi1mLu7i1KXS

Underfitting and Overfitting in Machine Learning

Cross-Validation - MATLAB & Simulink

Overfitting vs Underfitting in Machine Learning: Everything You Need to Know

Overfitting and Underfitting. Before getting to the topic, let's see…, by Ashish Gusain, Analytics Vidhya

Understanding Overfitting and Underfitting in Decision Trees, by Gunjan Gohain

Overfitting & Underfitting in Machine Learning - Analytics Yogi