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Assessing the Impact of Additional Evidence or Information. When arguing and losing, people often bring in extra evidence. “Ah, but what about…. ….the the example with the fruit bat?” he countered. We need to be able to detect what effect any extra evidence has on an argument.

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Assessing the Impact of Additional Evidence or Information

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Assessing the Impact of Additional Evidence or Information

When arguing and losing, people often bring in extra evidence.

“Ah, but what about….

….the the example with the fruit bat?” he countered.

We need to be able to detect what effect any extra evidence has on an argument.


The conclusions which we draw on the basis of evidence can sometimes be called into question when new information comes to light.

One way in which a person's reasoning may be challenged by others is by their presenting some relevant piece of information to which the person needs to respond.

If the piece of evidence supports one of the reasons or the reasoning then it strengthens the argument. If it supports an opposite conclusion it weakens the argument. It may be neutral or irrelevant and neither strengthen nor weaken the argument.

Example 1

One of the three people accused of this robbery is likely to get a longer prison sentence than the others, because he has a previous conviction for theft.

Additional statement

The judge in this case is known to be hard on first offenders in order to set an example

Is there an assumption underlying this argument?


There is an assumption underlying the argument and that is that judges generally give shorter sentences to first offenders than to those who have a previous conviction for robbery.

But the additional statement gives us reason to think that this particular judge will not act in the usual way because he wants to send a message to other potential robbers.

Hence it weakens the argument by suggesting that the judge may give just as long a sentence to a first offender as to a person who has a previous conviction.

Example 2

A vegetarian diet does not contain the saturated fats which are present in meat and which are so bad for health. So a vegetarian diet is very good for health.

Additional statement

Vegetables are high in certain vitamins which are essential to a healthy diet.


The argument here gives just one reason for thinking that a vegetarian diet is healthy – the reason being that something which is bad for health is excluded front the vegetarian diet.

The additional statement gives evidence that a vegetarian diet contains something which is positively beneficial for health, so it strengthensthe argument.

Assessing Extra Evidence.

Which would most weaken the argument?

A Polar bears are especially ill suited to a life in captivity.

B Many polar bears in the wild engage in obsessive patterns of behaviour

C Polar bears in captivity are much better fed than those in the wild.

D Polar bears in the wild cover many miles when hunting for food.

E. Polar bears that have been reared in captivity are incapable of surviving in the wild.

Polar bears in captivity frequently engage in obsessive patterns of behaviour, pacing back and forth on the same spot, swinging their heads from side to side and other signs of stress. They do this even when their living areas are spacious. What this shows is that conditions of captivity are not a satisfactory substitute for the natural environment of the polar bear species.

Assessing Extra Evidence.

A No mouthwash can be completely effective in preventing tooth decay.

B Tooth decay can be caused by eating too much sugar

C Teeth are more at risk of gum disease than they are from plaque.

D Only twenty people took part in the tests

E The people who took part in the tests did not use toothpaste when brushing their teeth.

A substance called plaque is the biggest cause of tooth decay. Most toothpastes contain something which attacks plaque, but tests have shown that a new mouthwash is much more effective than brushing in getting rid of plaque. Therefore, to reduce the risk of tooth decay, we should use this mouthwash rather than toothpaste.

Which would most weaken the argument?

Many cyclists use masks against city air pollution. The filters in the masks are efficient against pollutants, but only for approximately 30 days. Therefore users of such masks who wish to minimize the risks to their health from pollution should change their masks regularly.

Assessing Extra Evidence.

  • The carbon fibres in the masks are very cheap to make.

  • City cyclists are more prone to illness caused by pollution than are country cyclists,

  • The mask cannot offer protection against carbon monoxide, a pollutant which is prevalent in cities.

  • After 30 days’ use, chemicals absorbed by the mask can cause greater inhalation of pollutant than not using a mask.

  • Only one manufacturer of masks provided instructions about when to change the filter.

Which of the following,

if true, most strengthens

the argument?

A Many children can swim before they go to school

B Swimming has been shown to be an excellent form of exercise

C A large number of swimming pools have been opened in the last few years.

D There has been an increase in the number of adults taking swimming lessons

E The number of children learning to swim outside school hasdeclined in recent years

It has been suggested that the ability to swim is a crucial factor in reducing the risk of drowning and that therefore all school children should receive swimming lessons. However, the number of children drowning has dramatically decreased in recent years with the reduction of swimming lessons in schools. Most of those who drown do so in in open water where even an expert indoor swimmer cannot cope with the debilitating effects of cold water and strong currents. Non-swimmers, being naturally more cautious, are less likely to place themselves at risk near open water. Therefore, although swimming ability could save a life in optimum circumstances, an appreciation of the dangers of water is more likely to reduce the number of drownings than swimming lessons in schools

Which of the following, if true most strengthens the argument?

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