This chart shows the occurrences of defects in trees that failed (or broke in some way) during the April storms. These findings are from my own casual research:
Defect Visible - this refers to the trees that had a visible problem that indicated the tree was a risk
Defect Discoverable - these trees had a visible problem, but the problem was only identifiable through a full inspection and risk assessment
Defect Not Visible - these trees had a serious problem that predisposed them to failure, but the problem would not have been easily discovered, even by a professional. This includes problems such as root decay below the surface, or internal wood decay without external symptoms.
No Defect - this indicates that the tree did not have a predisposing defect that is normally associated with failure.