The reason you use a true experimental design is that you want to have the best chance you can to determine causality. But sometimes you can't conduct a true experiment. This doesn't mean that no research or useful information can be gathered. Several techniques exist that are useful, such as:

• Quasi-experimental designs
• Time Series
• Single-Subject Designs
• Survey Research
• Meta-analysis

These techniques do not involve direct manipulation of all postulated causal factors and this must give you caution as you report your conclusions or recommend policy decisions. However, used judiciously they are useful and can sometimes suggest true experimental designs.