"eBird is a true crossroads between birding and science. As we develop eBird, we're continually walking the line between building better tools that birders want to use, while maintaining our focus on collecting useful scientific data in the process. The truth is, every piece of data submitted to eBird is valuable--from single records of a bird in space and time (Incidental Observations), to complete checklists with associated effort information. But there are differences between the levels of analysis we can perform using the effort-based observations versus incidental observations. As eBirders, we're always interested in how to make the most out of our data, and in this article we'll explore some of the reasons why recording effort can make a big difference when it comes to data analysis."
Link: Keep reading at eBird.org