When can you Selectively look at Less than all of the Data?

I pass along Jason Brownlee's links from time to time, have found them very useful.   Subscribe to his stream, buy his books.  Here its about sampling and when you can look at less than all of the data. 

Jason @ ML Mastery jason@machinelearningmastery.com 
Thu, Jan 23, 1:12 PM (3 days ago)    to Franzdill

Hi, this week we have a tutorial on undersampling algorithms for imbalanced classification, a tutorial on combining oversampling and undersampling, and a tour of data sampling methods.

Discover how to delete examples from your dataset to improve performance:
>> Undersampling Algorithms for Imbalanced Classification

Discover specialized techniques that perform both oversampling and undersampling:
>> Combine Oversampling and Undersampling for Imbalanced Classification

Discover a suite of data sampling techniques available for imbalanced classification:
>> Tour of Data Sampling Methods for Imbalanced Classification

See his new book
Imbalanced Classification with Python
Better Metrics, Balance Skewed Classes, Cost-Sensitive Learning  ... "