. why was it useful to be able to verify the source of the water? . why would a neighborhood served by two diff erent water companies be more useful for testing snow's hypothesis than two neighborhoods each with their own source? . epidemiologists often draw causal webs to illustrate the interrelationships among biological, social, and environmental variables that contribute to disease outbreaks. based on what you have learned so far, what variables should be included in a causal web for cholera? . snow considered his conclusions about cholera to be inferences from observations whereas the reviewer from the medical journal considered these to be conjectures. what is the diff erence between inference and conjecture?

1. Private sources of water supply should be tested at least twice a year for total coliforms and E. coli, especially where sources of supply are most likely to be present. risk, ie after the spring thaw and during the rainy autumn period. Analyzes should be performed even if water treatment devices are installed on the water system.

In addition, the water should be subject to additional analysis following any incident that could affect the microbial safety of the reserves, such as sewage backflow or flooding near an area. well. Water should also be analyzed if it changes appearance, taste or smell.

2. To test Snow's hypothesis it would take at least two different water sources, and so the neighborhood would have to be served in water by two different companies.

3. Cholera, caused by the pathogenic strains of the bacterium Vibrio cholerae, is a highly contagious disease, with at least 100,000 cases reported in 2004, mainly on the African continent. The indigenous nature of V. cholerae in the aquatic environment reminds us that this bacterium, before being pathogenic for humans, plays an ecological role in its ecosystem. This latter property implies a direct influence of environmental and climatic conditions on the presence, persistence and abundance of the bacterium in the aquatic ecosystem and thus indirectly on the emergence of cholera in humans.

The main factors favoring outbreaks are of social origin:

the socio-economic level and the living conditions of the populations: poverty, low level of hygiene, difficulties of water supply;

natural conditions and risks: water warming (El Nino phenomena strongly correlated with outbreaks, climatic changes), natural disasters causing floods and corporate disruption (cyclones, earthquakes), etc.

geopolitical environments: armed conflicts, civil wars leading to refugee camps and precarious housing, lack of governance of institutions that limit access to drinking water resources and control of the environment.

4. Inference is a movement of thought from principles to conclusion. It is an operation which makes it possible to pass from one or more assertions, statements or propositions affirmed as true, called premises, to a new assertion which is the conclusion.

A conjecture is an assertion for which no demonstration is yet known, but which is strongly believed to be true, in the absence of a counter-example.

A conjecture can be chosen as a hypothesis or postulate to study other statements.

When a conjecture is demonstrated, it becomes a theorem and joins the list of facts.

==> In some ways, they are opposed in their deduction, the inference starts from principles and the conjecture starts from "conclusions" or hypotheses supposed to be right.

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