However, anyone planning to use an “experimental” design to test a hypothesis must plan the research in detail at the start, and so the techniques of data collection and analysis need to be firmly established before any data is collected. Research claiming to use this type of methodology, which involves substantive changes (in such matters as how “subjects ” are assigned to groups, or which involves statistical tests are to be applied ) once the research is underway, is open to being challenged as failing to follow accepted procedures and so is potentially invalid.