Estimation Techniques

Estimation Techniques along with Test Plans and Test Strategies are done throughout the Planning phase of testing.  First there are 2 techniques that ISTQB mentions, consulting people and analyzing metrics.  The consulting technique involves working with experienced staff members and testers drawing upon their collective wisdom to develop structure for testing the project. Then collaborate to understand, tasks, effort, duration, dependencies, and resource requirements that the tests will need to be successful.


Analyzing metrics can be as simple or sophisticated as the tester can make it. ISTQB states the easiest approach asks, 'How many testers do we typically have per developer on a project?'
A more reliable method classifies the project in terms of size (small, medium or large) and complexity (simple, moderate or complex) then sees the average length of projects of a particular size and complexity have taken in the past to test.
Another reliable yet easy approach  that ISTQB mentions is to look at the average effort per test case in similar past projects and to use the estimated number of test cases to estimate the total effort. The more sophisticated methods that ISTQB mentions involve building mathematical models in a spreadsheet that look at historical or industry averages for certain key parameters number of tests run by tester per day, number of defects found by tester per day, etc. and then plugging in those parameters to predict duration and effort for key tasks or activities on your project. The tester-to-developer ratio is an example of a top-down estimation technique, in that the entire estimate is derived at the project level, while the parametric technique is bottom-up, at least when it is used to estimate individual tasks or activities.
ISTQB testers prefer to start by drawing on the team's wisdom to create the work-breakdown structure and a detailed bottom-up estimate. We then apply models and rules of thumb to check and adjust the estimate bottom-up and top-down using past history. This approach tends to create an estimate that is both more accurate and more defensible than either technique by itself.
Even the best estimate must be negotiated with management however. Negotiating sessions exhibit amazing variety, depending on the people involved. However, there are some classic negotiating positions. It's not unusual for the test leader or manager to try to sell the management team on the value added by the testing or to alert management to the potential problems that would result from not testing enough. It's not unusual for management to look for smart ways to accelerate the schedule or to press for equivalent coverage in less time or with fewer resources. In between these positions, you and your colleagues can reach compromise, if the parties are willing. Our experience has been that successful negotiations about estimates are those where the focus is less on winning and losing and more about figuring out how best to balance competing pressures in the realms of quality, schedule, budget and features.