How to Determine a Factor of Safety

Factors of safety should be determined based on the error in the previous estimation and then adjusted as needed. Even if a process does not use measurements to arrive at an estimate, a factor of safety can be established for future similar estimates. For example, a test effort was estimated to require 14 weeks. In reality, 21 weeks were required to complete the test effort. The estimate was low by a factor of:

21/14 = 1.5

When the next test estimation effort takes place, if the same or similar methods are used to make the estimate, even if it is based on an I-feel-lucky guess, multiply the new estimate by a factor of 1.5, and you will get an estimate that has been adjusted to be in keeping with reality.

Note	It does not matter how a factor of safety is determined; using them improves estimates.

Always Get a Second Opinion

No good reason exists for working without a safety net. Inspection and formal reviews are the most productive way to remove defects that we know of today. Part of the reason is that inspectors bring an outside perspective to the process, so the rest is simple human factors. Having someone to check your work is very important.

Note	Two points for engineers in software to keep in mind: 1. You need to pick the right tool method or approach for the job at hand. 2. You need to remain flexible and plan for change.

Testers do not have to be experts in a system to test it well. They must be trained testers, using a systematic approach, sound reasoning, and measurements to make their case. They must also be well informed about the project and have good channels of communication to the experts. People skills are also a definite plus.

Like it or not, good testing also includes exploration. Good testers are the ones that dig in and explore the system. They want to know how it works. The ones that don't dig in aren't going to get the job done, and the ones that think they know how it works are no good either, because they are closed and biased.