We have located the results of empirical studies on elicitation techniques and aggregated these results to gather empirically grounded evidence. Our chosen surveying methodology was systematic review, whereas we used an adaptation of comparative analysis for aggregation because meta-analysis techniques could not be applied. The review identified 564 publications from the SCOPUS, IEEEXPLORE, and ACM DL databases, as well as Google. We selected and extracted data from 26 of those publications. The selected publications contain 30 empirical studies. These studies were designed to test 43 elicitation techniques and 50 different response variables. We got 100 separate results from the experiments. The aggregation generated 17 pieces of knowledge about the interviewing, laddering, sorting, and protocol analysis elicitation techniques. We provide a set of guidelines based on the gathered pieces of knowledge.