Ecological studies have acquired a bad reputation as weak scientific studies suffering from the ecological fallacy and many intractable limitations of data and method. In order to reduce these problems, it is suggested that analyses be focused on specific populations-at-risk, diseases, risk factors and places. In addition, seven guidelines are suggested which will limit the possibility of false positive results and provide the best clues for expensive, follow-up research. These guidelines are testing to make sure that the results are consistent across different times, places and methods. The guidelines were applied to finding the best clues for occupational-related cancers among white males aged 35-64 in the State of Illinois during 1950-1975. The most interesting clue found was that most coal mining areas in Illinois during the late 1960s and 1970s manifested the highest cancer mortality rates of trachea, bronchus, and lung and among the highest rates of increase of this type of cancer. This finding was unexpected because many studies have shown low lung cancer rates among coal miners.