An article headline claimed that "drugs cause homelessness" due to a positive relationship found between homeless populations and drug use. educated psychologists thought this might be flawed, because they thought unemployment was influencing both drug use and homelessness. this is an example of:

The answer is a third variable.

A third variable or a confounding variable is a variable that influences two separate variables that are being measured. In this case the two separate variables being measured are drug use and homeless populations, the third variable here would be unemployment.

development:   floodplain development often directly impacts wetlands by removing vegetation (increasing bank erosion),   and filling or draining wetlands for building sites.   floodplain development sometimes indirectly impacts riparian wetlands through the installation of artificial stream stabilizing devices like rip-rap and bulk-heads that attempt to stop the natural meandering process which creates new wetlands and replenishes existing ones.    

road building:   most river valleys have roads and/or railroads.   these structures squeeze rivers and streams by narrowing the floodplain.   this destabilizes the river which   has less room to meander and therefore has an excess of energy.   the roads and railroads also affect drainage from uplands onto the floodplains, and many are built on top of areas that once were wetlands.   such roads often create long, low-quality wetlands upslope of the road by interrupting surface and groundwater flows.   these wetlands can attract wildlife dangerously close to roads.   wetland loss caused by roads is mitigated through the restoration of other impacted wetlands although the replacement wetlands are not always of the same type and quality as those lost. see montana department of transportation's off-site wetland mitigation reserves map.

grazing: overgrazing harms wetlands through soil compaction, removal of vegetation, and stream bank destabilization.   wetlands offer some of the best forage for livestock as well as a water source and cover, so livestock tends to spend a disproportionately large time in wetlands.   there are many grazing strategies that discourage cattle from using wetlands. click on rwrp's web site for a discussion of these strategies.

agriculture:   wetlands often have fairly flat areas of rich organic soil that is highly productive agricultural land if drained.   for this reason many wetlands have been drained and converted to agricultural lands.  

mining: historic mining has had a major impact on wetlands of the clark fork basin, particularly along the upper river where mine wastes have been deposited in the floodplain, creating the country’s largest superfund site. view the superfund site.  

the epa also list the following as major human causes of wetland loss:   logging,   runoff, air and water pollution, introducing nonnative species. see examples of nonnative species here.

the following lists specific damaging actions commonly taken in wetlands.

dumping:   dumping fill material buries hydric soils and effectively lowers the water table so hydrophytic (water loving) plants cannot compete with upland plants.  

            dredging:   the removal of material from a wetland or river bed.   dredging of streams lowers the surrounding water table and dries up adjacent wetlands.

            draining:   water is drained from wetlands by cutting ditches into the ground which collect and transport water out of the wetland.   this lowers the water table and dries out the wetland.    

            diverting flow:   water is diverted around wetlands, lowering the water table.  

            devegetation:   vegetation plays an important role in wetland ecology by removing water through evapotranspiration, altering water and soil chemistry, providing habitat for wildlife, and reducing erosion.   removal of vegetation can drastically and sometimes irreversibly alter wetland function.

            damming flow:   many ponds and reservoirs are constructed on wetlands.   a flooded wetland cannot provide the same habitats and functions.  

            development of springs:   pumping large quantities of water from springs lowers nearby groundwater and can result in the loss of wetland vegetation.   compaction of springs by cattle can cause springs to cease to flow.

explanation:

what are the answer choices?