Hilda is studying the effects of a major natural disaster on people living nearby. She finds that many of the victims are depressed and show stress-related symptoms. Why can't Hilda conclude that the natural disaster caused the depression and stress-related symptoms

She does not have any comparison group

Explanation:

Comparison group is cluster of people in form of group that is used for evaluating and comparing task performed so that working condition of experiment can be known. Comparison group is subjected with same condition and equipment to measure progress and impact of experiment.

According to the question, Hilda does not have comparison group with whom she can actually compare that natural disasters effect on people as stress and distress.She cannot evaluate the condition of people without any similar reference.

i thought a 14 year old should know this but

colonies   were tied to britain through trade and by the way they were governed.   trade was restricted so the colonies had to rely on britain for imported goods and supplies.   there were no banks and very little money, so colonists used barter and credit to get the things they needed.  

britain also needed money to pay for its war debts.   the king and parliament believed they had the right to tax the colonies.   they decided to require several kinds of taxes from the colonists to pay for the french and indian war.   these taxes included the stamp act, passed in 1765, which required the use of special paper bearing an embossed tax stamp for all legal documents.   other laws, such as the townsend acts, passed in 1767

this mainly outraged the colonies and created what we know as today as the american revolutionary war

the invasive species was introduced due to commercial purpouses, as there was a demand for fur clothes and hence, a business opportunity connected to this product. actually, it was when the nutrias scaped, causing harm to the us ecosystems when they acquired the status of invasive species. in this case they have caused an environmental problem, but it could have affected also economic interests or even human health.