274 Unit 7 Critical reasoning: Advanced Level
lacking from the experiment is a second group
in which prison sentences are reduced, or held
at the same level, to see what effect that has. If
this, the ‘control’ group, shows no reduction
in crime, then it would support the case for
the effectiveness of prison. But if the outcomes
in the control group were the same as in the
main group – or even resulted in a bigger
reduction – then the argument that prison
works would be severely weakened.
Obviously one whole population cannot be
subjected to both experiments, main and
control, at the same time. But different regions
with different crime-fighting policies can be
compared. Similarly, different periods in
history, when different methods were in
operation, can also be compared. Chart 5 on
the next page is an example of such a
comparison.Activity
Comment critically on the statistical
information in Chart 5 and the claims made
on the strength of them.
Can the following claim, from the headline of
the document, reliably be inferred?
[4] We can be safer when we imprison
fewer people.(Keep in mind what you already know from
Charts 3 and 4.)Commentary
Clearly this bar chart is intended to counter
the claim that prison works. As they stand, the
statistics are impressive. Over the ten-year
period from 1999 to 2009, when imprisonment
was rising generally across the USA and crime
falling, the state of New York saw a reduction
in its prison population and an accelerated fall
in crime, compared with the state of Indiana
which had a huge rise in its prison population
and a much smaller fall in crime.have realised that [3] makes this assumption.
Surely it is just as plausible that the causal
connection is the reverse: that an explosive
rise in crime has pushed the prison
population higher and higher. If so, it is
crime that is ‘working’, and the slogan should
be: ‘More crime, more prison!’
Why would reported crime then fall as it
did from 1991? Well, there are plenty of
possible reasons. One is that the police may
have become better at solving crimes, and that
conviction rates have risen accordingly. That
would reduce crimes and result in more
criminals going to jail, and thus explain both
graphs. It would not mean that prison was
working, but that detection and prosecution
were working. It is possible, too, that there was
merely a reduced rate of reported crime, or a
change in the way crime is classified and
recorded. That sometimes happens as
politicians try to reassure the public that the
fight against crime is being won, and they
have less to fear. So long as there are other
plausible ways in which the trend in Charts 3
and 4 can be explained, the claim that prison
is the driving force is weakened.
Be careful, however, not to swing too far in
the other direction, towards option C. Doubt
about the support that the statistics give to [3]
does not mean that [3] must be false. In fact
the data that can be read off from the graphs
gives no more support to the claim that prison
does not work than to the claim that it does.No control group
Another way in which statistical information
may mislead is by giving only one side of the
picture. What is missing from the data is what
researchers refer to as a control group. If we
think of the period of time during which the
prison numbers rose as an ‘experiment’, we
can see what this means. The experiment was
performed on a whole population, and the
observed outcome was that as prison numbers
rose, crime figures rose and then fell. What is