To conclude this, is not an easy task. Judges are giving prison sentences to thousands of people in the Netherlands for a crime they have committed. They have several goals of which prevention is one of them (see sub-question 1 for further info about these goals). They try to avoid that these people will do criminal acts again. They try to reach this goal by putting them into captivity, specific prevention and re-socialization. We all know that goals sometimes can't be achieved. And it is hard to find out if a prison sentence is effective?
How do we find results?
First, we want to know which conditions are required to make a prison sentence effective or not. We used two different ways to find out if it is effective or not.
1. Group experiment research with different penalties.
When we want to measure the effect of prison sentences, we actually need to do an experiment of which one group of detainees does get a prison sentence, and another similar kind of group gets a different kind of penalty. There are some disadvantages about this experiment though, especially when it's about longer prison sentences, which might raise some ethical questions. Judges and especially prisoners won't think it is fair that they get a prison sentence, whilst others receive a different kind of penalty for the same crime.
2. Statistic research
By using specially designed statistic techniques, it is possible to recreate an experimental situation which comes really close to reality. This technique helps to create a proper comparisons group. For example comparing someone with a long criminal record that does get a prison sentence with someone with an evenly long criminal record which doesn't get that. Thanks to these research methods we can compare the criminal behaviour after their sentence with the results of the comparison group, just like with the experiment. We can also conclude that these differences found in the results can't be differences that were already there before the experiment.
There are two important institutions which investigate the relation between imprisonment and recidives: the NSCR (Dutch Studycentre Criminal studies and Justice) and the WODC (Scientific Research and Document Centre). They both come to the conlcusion that prison sentences increase the recidivism rate.
Statistic research shows the facts about the recidivism. A surprising outcome of these researches show that recidivism is quite high, after the detainees have left jail. You see that in de diagram below. It states the percentage of ex-detainees which will be convicted for a crime, and who get convicted again with a jail sentence. This is given in years after they have left jail. You see that the chances get higher throughout the years, up to about 75% of the ex-detainees that will be convicted again 7 years after they have left prison. grafiek percentage - years after detention.png
Diagram: percentage ex-detainees that get convicted again and will again receive a prison sentence
- as calculated in years after leaving jail
Source: DJI, Jaarverslag 2006, Ministerie van Justitie, The Hague.
The only malfunction of these statistic for our answer we want to have, is that it only shows factual information about the recidivism results. With only facts, you don't know the reasons why the recidivism is so high. It is very important to get to know the problem and to improve things, so the recidivism percentage gets lower.
In 2009, Wartna&Tollenaar did a research  , which looked at different penalties and the chance of recidivism with each penalty. They made three categories: a fine, community service and a prison sentence. Results showed that 23% of the people who got a fine got convicted by justice within two years. This percentage was 26% of people who received a community service. But the recidivism percentage of people who got a prison sentence was 50%! This shows us clear that people who receive a prison sentence have a higher chance to get convicted by justice. Now you know the facts, but does this give you information to assume that prison sentences actually cause this high recidivism? No, not in general. So for more information about the cause of recidivism, you have to do a different kind of research. Another thing which is doubtful, are the people who get fines as a conviction really the same as people that get a prison sentence as a conviction? Or does the judge take into account that for the same criminal offence, some people are more likely to return in their old criminal behaviour after the conviction, than others. People who have a small chance of recidivism are more likely to receive a fine.
Theories about possible causes
To get to know the reasons/explanations for the outcome of the statistic recidivism results, there are several theories about criminal recidivism  .
Prison is a school for criminal behaviour
When a prisoner 'lives' inside jail, he meets all kinds of criminals. This is a cooking bowl, which might produce and stimulate criminal behaviour. This is the place where all the people with criminal behaviour are around, and it is easy to think and assume that the their thoughts will build up together.
The scare-off effect is inadequate
What our detainee in our interview also mentioned, is that prison sentences don't scare off as much as we want to. He said that this will only scare off people who never been inside jail, but once you know how to live and survive inside jail, it doesn't scare you anymore.
Loss of house/job/family
Although the government tries to avoid this, that's what they say, it is inevitable that employers will fire employees who have to stay in jail for a while. Also your reputation and image gets brutally damaged when you spend quite some time in jail. The social image is not very positive about that, and that will affect
Prisoner gets ego-centric and asocial
You have to survive inside jail. It isn't a social centre where a lot of social contacts are available. This makes the prisoners automatically more selfish, and want to survive. To survive in jail, you sometimes have to be harsh and asocial. When they return into society, which has to do a lot with proper social contacts, they don't know how to handle anymore when around with other people.
The social and educational level of intelligence of prisoners
Mainly, the type of people who commit crimes such as burglary, theft and physical conducts, are the lower class people in society. They often haven't had a high education level and are not capable to see the possible outcomes of their criminal behaviour. Of course there will always be odd ones out.
Social network of detainees
Family, relatives and friends of criminal people will have a high chance of not accepting the criminal behaviour and will decrease or stop their social contact with the detainee. This will put the detainees into a social isolation, which produces a high chance of recidivism.
Families who don't change their social contact towards the detainee can sometimes approve the criminal behaviour and might even encourage this.
The six theories mentioned on the previous page can have an influence/relation on the course of life, and the possible future criminal behaviour of the criminals. You can see this relation in the figure below 
grafiek relatie gevangenisstraf, levensloop omstandigheden en crimineel gedrag.png
Theories also can contradict each other  . For example with learning theories:
+ when a detainee learns to control and decrease their criminal behaviour by attending anti-aggression classes, do a professional training and learn how to handle within a work-society, they will be more prepared for life outside detainment. This will decrease recidivism.
- detainees can't work throughout their detainment, can't get new work experiences which are relevant for the labour market. They also can learn new criminal behaviour/techniques from other inmates. This all makes the chance for recidivism higher.
Another example with socialization theories:
+detainees will learn proper social behaviour from the DJI-employers, and take over their conventional behaviour, norms and values. This will decrease recidivism.
-detainees will get in contact with other criminal inmates, and take over their deviant value-orientations. This makes the chance of recidivism higher.
A research method that J.A. Blokland and D. Nagin used in 2007 (Nieuwbeerta, P.A. Blokland, & D. Nagin. De effecten van gevangenisstraf op het verloop van criminele carrières. Mens en Maatschappij, 82, 272-279)  was looking at test-groups A real life experiment with criminals would be problematic, so therefore they used a specially made statistic technique to create an experimental situation, which comes as close to real life as possible. This technique helps to compose a proper matching group. They compared for example someone who got a prison sentence who already had a criminal record, with someone who also received a prison sentence who didn't have a criminal record yet.
They looked at criminal behaviour of the prison-sentenced people after their detention period with the control-groups. Therefore we can be sure that the results found with this method aren't caused by differences that were already there before the jail conviction of the judjge.
In this research, they looked at more than 5000 people who got in touch with justice in 1977 for a criminal act. They composed two groups: one group with people who received a jail sentence between the age of 26 and 28. The other group with people who had the same age, who also got in touch with justice for a criminal act, but didn't get convicted or they received a fine or had to do community service.
After that, they looked at people from both groups who were alike, based on their criminal record (e.g. the type and amount of earlier convictions), personal characteristics (e.g. gender, origin) and civil class (e.g. married, children), so that they had about the same chance of getting in touch with justice. In total they compared 138 people with a jail sentence with a group of 336 people who didn't receive that jail sentence.
The conclusion of that research was that the people with prison sentences, on average more often got in touch with justice than people from the control-group. They got in touch with more new criminal acts as people did who didn't receive a prison conviction. Even serial-crime committers (veelplegers) get more in touch with justice when they had received a jail sentence than people who didn't. So in comparison with non-prisoned people, the chance of criminality gets higher instead of lower, when they are convicted to a jail-period.
Looking at the two different approaches (statistics research and experiment research) we can make a conclusion about our question. Seeing both results, you can say that prison sentences are not effective. Hard statistics show that the recidivism will get even up to 75% within seven years after their release from prison. Other statistics showed that the recidivism is almost twice as high when an offender gets a jail sentence instead of a community service or a fine. Also the experiment research mentioned, done by Blokland and Nagin, showed us that the recidivism of jail sentences is higher compared with people who received a different penalty.
By seeing that these facts and experience show the same end result, and noticing that this gives us a as much fair view of reality as possible, we conclude inevitable that the recidivism is higher when a criminal receives a jail sentence.
This is a serious conclusion. When a judge takes this into consideration, it can influence his justicial decision. Though he has to work according the law, this might indirectly influence him. Nonetheless the judge we interviewed said that he would only see a jail sentence as a final solution to penalize criminals after looking at other punishments. Though it is not morally possible that judges can base their decision just on our conclusion such as we made in this sub-question or made by other researches.
You can compare this with a medical example: when a doctor has many patients waiting in his waiting room to visit him. In his medicine cabinet, he only has only has one medicine to cure these people. Though he knows that this medicine isn't reliable, if it has bad side effects, so it can work positively or negatively. He doesn't know the final outcome of that medicine with each patient.
This situation wouldn't be tolerated in a medical context: doctors cannot just prescribe medicines of which isn't adequately proved that they have a positive cure of these patients.
With the knowledge we know, you can say that a prison sentence is comparable with an alternative medicine (e.g. acupuncture) - of which many people think it has a positive outcome, but of which is it never officially proven that is really positive.
DJI, Jaarverslag 2006. Ministerie van Justitie, Den Haag.