Experimental research designs aim to identify objectively true causal relationships between variables. They involve a determination of correlation, but also attention to temporal order and elimination of potential alternative explanations in order to establish causality. Sample size is not a major concern; there is no need for probability sampling, and any form of measurement can be used. They are always longitudinal in their orientation to time. Results are always numerical. Experimental research contributes to HDFS by providing evidence of causation.