Unobtrusive methods are archival records where one is given the opportunity to assess the impact of natural events and examine other issues. They are strong on external validity because of the subjects unawareness of the research or its aims. When using archival records, we must control for the possibility of internal invalidity or spurious relationships. Archival research may require the collection of additional data and temperature records. A significant issue in archival research is construct validity and reliability of the data for research purposes. Archives are subject to gaps and incompleteness that make it difficult to determine whether the data, which was available, represented the population, which was of interest. The use of archives is economical, the...