> For the complete documentation index, see [llms.txt](https://apphp.gitbook.io/artificial-intelligence-with-php/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://apphp.gitbook.io/artificial-intelligence-with-php/machine-learning/data-fundamentals/ml-data-processing/stages-of-data-processing/typical-problems-with-data.md).

# Typical Problems with Data

Typical problems with data are high cost, bias, low predictive power, outdated examples, and leakage.

Reasons for data leakage Let's discuss three most common reasons for data leakage during data collection and preparation:

1\) the target is a function of the feature;

2\) the feature hides the target;

3\) the feature is taken from the future.