One effective activity which many organisations carry out to prevent defect
from reoccurring is Causal Analysis and Resolution. (CAR)

The purpose of carrying out CAR activity is to:

1) Identify the root causes of a defect.

2) Come up with an action plan to work to its closure and adhere to it so that
such defects don't recur.

Tools/Techniques to carry out CAR:

Brainstorming: A meeting is organised where all the members come up
wth the root causes. The solutions are collected and then most feasible solution
is again brainstormed for the feasibility and correctness.

Pareto Analysis: A graph is drawn to classify the root causes of each
of the defects found. Pareto is based on 80/20 rule which depicts that 80 of
defects are caused by 20 of causes. These 20 of the causes can be looked at on
priority.

Fishbone Diagram: All possible causes grouped into People Process
Technology Resources and Mother Nature. Ideas from different members are
captured. The root cause is further drilled down (Doing further 'Why Analysis').
Appropriate remedies are put into place finally.

Another way to prevent defects is to have a strong review mechanism in place
right from the early stages of testing/development cycle so that the issues are
not migrated in later stages.

QA will not prevent defects from occurring but can minimize the quantity and severity of defects by:

1) fully understanding the scope of the software development project
2) getting involved at the earliest possible stage in the development cycle
3) reviewing the project plan for development and offering feedback and
4) maintaining a presence in the development process before the first release to test.