First you need to define "effectiveness". Do you mean effective at finding bugs, effective at proving requirements satisfaction, effective at code coverage, something else, or all of the above? And then by "effective" do you mean thorough, efficient, non-redundant, something else, or all of the above.
"Out of 1000 test cases 850 passed and 250 failed, who effective is the test area? " Well, all I can ascertain from this is that there is a problem with the effectiveness of your test case pass/fail accounting (unless these number are not base 10?). [img]images/icons/wink.gif[/img]
Another way of measuring effectiveness of test cases would be the number of defects detected using the test cases which have been structured to the total number of defects which also include the defects reported by customers/end users.