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By comparing the genetic differences between HIV-2 strains , the researchers estimated that the virus was transmitted to humans in the first half of the twentieth century , when sooty managbeys were more common in West Africa . GIS-based environmental modelling activities with complex data requirements were rare , and simple interpretations based on classifications of single data layers were more common . Here , red-tailed hawks were more common than in the forests , but keep in mind that prairie grasses were eight to ten feet tall . Sperm whales were more common in summer than winter by a factor of roughly two , and occurred less often at the westernmost site surveyed ( 52N , 157W ) than elsewhere in the Gulf . PTSD and depression were more common among those who said their smoking and drinking increased ( Galea et al ., 2002 ) .