Objective tests of knowledge or skill are desirable for their objectivity. However, the construction of valid tests is quite difficult. Physiological measures are also prized for their objectivity, but they tend to require expensive equipment, are invasive, and demand expert interpretation. Several types of archival records are used in HDFS, including public administrative records (of things like births, marriages, and custody), school and medical records, and big data from consumer behavior (including things like mobile phone geolocation). All have been useful in HDFS scholarship. Archival records can be used in very large studies and sometimes do not require consent of the subjects so they eliminate non-response bias in the study samples, but the scientist lacks control over these measures, and so they are dependent on the quality of the original data. Finally, content analysis is the analysis of cultural artifacts in order to understand cultural phenomena, which is an indirect measure but it can provide insights that are otherwise hidden from awareness for most people.