Defect metrics - open and close rate explanation

Question

Probably I am missing some key aspect here, because I simply have no idea how this metric is supposed to tell me anything described by the authors:

Defect Open and Close rate = (Defects found during testing in a release / (Defects found during testing in a release + Defects found after release) *100

What is it?

While there are numerous defect statuses which indicate the current state of an identified flaw, they can typically be categorized as either open or closed.

Why is it important?

The sheer number of defects that are encountered during a project’s run can make them difficult to keep tabs on. If team members are not diligent about measuring the current status of their program’s flaws, certain defects could slip through the cracks and show up in the finalized release. Furthermore, comparing the frequency of open defects with close rates will also provide insight into the ability of testers and developers to work together to identify and address software issues.

http://www.getzephyr.com/resources/whitepapers/qa-metrics-value-testing-metrics-within-software-development

Answer 1

The ratio "defects found by QA before release / total defects found" is a measure of testing effectiveness. If many defects are discovered after QA has OK-ed it, they apparently didn't test it sufficiently. Why that is the case may have a myriad of reasons. Wasn't there enough time for thorough testing? Were there unclear requirements? Were the testers not sufficiently familiar with the subject matter? A low rating can invite reflection on the causes.

This ratio is important because the raw number of defects found is meaningless. If the testers didn't find anything, did that mean they didn't look? No, it could also mean that the system is perfect (lol). And if they find many bugs, does this mean they found all bugs? No, they could still have overlooked many more problems.

The disadvantage of this metric is that it can only be calculated retrospectively, a long time after the release.

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The text accompanying this formula seems to focus on something entirely different, and I'm not sure what. Note that the formula does not contain any close rate, and only compares in which project phases defects are found (QA vs. post-release). I will assume the authors accidentally combined that formula with this text when formatting their article.