I just read the article "Thirsty World" on Science News. 08/18/2018, Vol. 194 Issue 4 p14-17. It is a sample study about the world's safe water and sanitation. I know it is an observational study. I am stuck trying to figure out what type of sample it is. Census, systematic, cluster, stratified, convenience, or random are the sample types to choose from. Can anyone help?