incidence

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The Word of the Day for January 19 is:

incidence   \IN-suh-dunss\   noun 
     : rate of occurrence or effect

Example sentence: 
     "When ... water flows through a system with a lot of lead 
in its plumbing, you'd expect to find a high incidence of lead 
contamination. That's exactly what our tests showed." 
(_Consumer Reports_, February 1993) 

Did you know?
     The words "incident," "incidence," and "instance" may seem 
similar (and, in fact, "incident" and "incidence" are closely 
related), but they are not used identically. In current 
use, "incidence" usually means rate of occurrence and is often 
qualified in some way ("a high incidence of crime"). "Incident" 
usually refers to a particular event, often something unusual 
or unpleasant ("many such incidents go unreported"). "Instance" 
suggests a particular occurrence that is offered as an example 
("another instance of bureaucratic bumbling"); it can also be 
synonymous with "case" ("many instances in which the wrong 
person was arrested"). The plural "incidences" sometimes occurs 
in such contexts as "several recent incidences of crime," but 
this use is often criticized as incorrect.