Why is it ineffective to estimate testing effort solely based on a percentage of development effort?

Estimating testing effort solely based on a percentage of development effort is ineffective primarily because the quality of the development estimate may be poor. Development efforts can be miscalculated for various reasons, including unforeseen complexities, lack of comprehensive specifications, or changes in scope during the development process. If the foundation of the testing estimate is built on an inaccurate or unreliable development estimate, it is likely to lead to inaccurate projections for testing resources and timelines as well.

A testing process requires attention not just to the code itself but also to the overall system behavior, including external factors like user interaction, performance under load, and integration with other systems. If the development estimate does not accurately reflect the real work needed, the resulting testing estimates will also misalign with the actual needs, potentially leading to either resource shortages or over-allocations.