Data quality
You can collect data from anywhere and from any source. Whether the source of your data is reliable and is providing you with quality data is something you should work toward. If the source of your data is garbage, then the output of your processing system will also be garbage. Garbage in, garbage out. For example, if you are collecting data by scrapping different websites, you need to understand how to make sense out of it as you can't always assume that the data will be good quality. This may be useful in certain application use cases, but not all. Stringent data quality rules are usually applied to data that is used in compliance use cases. These quality rules can be applied at the collection level or at the processing level. Generally, these rules are spread across the collection and processing level. Collection Systems usually handle the first level of checking on the data quality, and then an underlying processing system does a more thorough quality check.