by Tom Rogers, Twitter Link
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Type I and Type II Errors - Making Mistakes in the Justice System


Ever wonder how someone in America can be arrested if they really are presumed innocent, why a defendant is found not guilty instead of innocent, or why Americans put up with a justice system which sometimes allows criminals to go free on technicalities? These questions can be understood by examining the similarity of the American justice system to hypothesis testing in statistics and the two types of errors it can produce. (This discussion assumes that the reader has at least been introduced to the normal distribution and its use in hypothesis testing. Also please note that the American justice system is used for convenience. Others are similar in nature such as the British system which inspired the American system)

True, the trial process does not use numerical values while hypothesis testing in statistics does, but both share at least four common elements (other than a lot of jargon that sounds like double talk):

  1. The alternative hypothesis - This is the reason a criminal is arrested. Obviously the police don't think the arrested person is innocent or they wouldn't arrest him. In statistics the alternative hypothesis is the hypothesis the researchers wish to evaluate. 
  2. The null hypothesis - In the criminal justice system this is the presumption of innocence. In both the judicial system and statistics the null hypothesis indicates that the suspect or treatment didn't do anything. In other words, nothing out of the ordinary happened  The null is the logical opposite of the alternative. For example "not white" is the logical opposite of white. Colors such as red, blue and green as well as black all qualify as "not white".
  3. A standard of judgment - In the justice system and statistics there is no possibility of absolute proof and so a standard has to be set for rejecting the null hypothesis. In the justice system the standard is "a reasonable doubt". The null hypothesis has to be rejected beyond a reasonable doubt. In statistics the standard is the maximum acceptable probability that the effect is due to random variability in the data rather than the potential cause being investigated. This standard is often set at 5% which is called the alpha level.
  4. A data sample - This is the information evaluated in order to reach a conclusion. As mentioned earlier, the data is usually in numerical form for statistical analysis while it may be in a wide diversity of forms--eye-witness, fiber analysis, fingerprints, DNA analysis, etc.--for the justice system. However in both cases there are standards for how the data must be collected and for what is admissible. Both statistical analysis and the justice system operate on samples of data or in other words partial information because, let's face it, getting the whole truth and nothing but the truth is not possible in the real world.

It only takes one good piece of evidence to send a hypothesis down in flames but an endless amount to prove it correct. If the null is rejected then logically the alternative hypothesis is accepted. This is why both the justice system and statistics concentrate on disproving or rejecting the null hypothesis rather than proving the alternative. It's much easier to do. If a jury rejects the presumption of innocence, the defendant is pronounced guilty. 

Type I errors: Unfortunately, neither the legal system or statistical testing are perfect. A jury sometimes makes an error and an innocent person goes to jail. Statisticians, being highly imaginative, call this a type I error. Civilians call it a travesty. 

In the justice system, failure to reject the presumption of innocence gives the defendant a not guilty verdict. This means only that the standard for rejecting innocence was not met. It does not mean the person really is innocent. It would take an endless amount of evidence to actually prove the null hypothesis of innocence. 

Type II errors: Sometimes, guilty people are set free. Statisticians have given this error the highly imaginative name, type II error. 

Americans find type II errors disturbing but not as horrifying as type I errors. A type I error means that not only has an innocent person been sent to jail but the truly guilty person has gone free. In a sense, a type I error in a trial is twice as bad as a type II error. Needless to say, the American justice system puts a lot of emphasis on avoiding type I errors.  This emphasis on avoiding type I errors, however, is not true in all cases where statistical hypothesis testing is done.

In statistical hypothesis testing used for quality control in manufacturing, the type II error is considered worse than a type I. Here the null hypothesis indicates that the product satisfies the customer's specifications. If the null hypothesis is rejected for a batch of product, it cannot be sold to the customer. Rejecting a good batch by mistake--a type I error--is a very expensive error but not as expensive as failing to reject a bad batch of product--a type II error--and shipping it to a customer. This can result in losing the customer and tarnishing the company's reputation.

Justice System - Trial
Defendant Innocent Defendant Guilty

Reject Presumption of Innocence (Guilty Verdict)

Type I Error Correct

Fail to Reject Presumption of Innocence (Not Guilty Verdict)


Type II Error

              Statistics - Hypothesis Test
Null Hypothesis True Null Hypothesis False

Reject Null Hypothesis

Type I Error Correct

Fail to Reject Null Hypothesis


Type II Error

In the criminal justice system a measurement of guilt or innocence is packaged in the form of a witness, similar to a data point in statistical analysis. Using this comparison we can talk about sample size in both trials and hypothesis tests. In a hypothesis test a single data point would be a sample size of one and ten data points a sample size of ten. Likewise, in the justice system one witness would be a sample size of one, ten witnesses a sample size ten, and so forth.

Impact on a jury is going to depend on the credibility of the witness as well as the actual testimony. An articulate pillar of the community is going to be more credible to a jury than a stuttering wino, regardless of what he or she says.

The normal distribution shown in figure 1 represents the distribution of testimony for all possible witnesses in a trial for a person who is innocent. Witnesses represented by the left hand tail would be highly credible people who are convinced that the person is innocent. Those represented by the right tail would be highly credible people wrongfully convinced that the person is guilty.

At first glace, the idea that highly credible people could not just be wrong but also adamant about their testimony might seem absurd, but it happens. According to the innocence project, "eyewitness misidentifications contributed to over 75% of the more than 220 wrongful convictions in the United States overturned by post-conviction DNA evidence." Who could possibly be more credible than a rape victim convinced of the identity of her attacker, yet even here mistakes have been documented.

For example, a rape victim mistakenly identified John Jerome White as her attacker even though the actual perpetrator was in the lineup at the time of identification. Thanks to DNA evidence White was eventually exonerated, but only after wrongfully serving 22 years in prison.

If the standard of judgment for evaluating testimony were positioned as shown in figure 2 and only one witness testified, the accused innocent person would be judged guilty (a type I error) if the witnesses testimony was in the red area. Since the normal distribution extends to infinity, type I errors would never be zero even if the standard of judgment were moved to the far right. The only way to prevent all type I errors would be to arrest no one. Unfortunately this would drive the number of unpunished criminals or type II errors through the roof.




figure 1. Distribution of possible witnesses in a trial when the accused is innocent

figure 2. Distribution of possible witnesses in a trial when the accused is innocent, showing the probable outcomes with a single witness.


Figure 3 shows what happens not only to innocent suspects but also guilty ones when they are arrested and tried for crimes. In this case, the criminals are clearly guilty and face certain punishment if arrested. 


figure 3. Distribution of possible witnesses in a trial showing the probable outcomes with a single witness if the accused is innocent or obviously guilty.. figure 4. Distribution of possible witnesses in a trial showing the probable outcomes with a single witness if the accused is innocent or not clearly guilty..



If the police bungle the investigation and arrest an innocent suspect, there is still a chance that the innocent person could go to jail. Also, since the normal distribution extends to infinity in both positive and negative directions there is a very slight chance that a guilty person could be found on the left side of the standard of judgment and be incorrectly set free.  

Unfortunately, justice is often not as straightforward as illustrated in figure 3. Figure 4 shows the more typical case in which the real criminals are not so clearly guilty. Notice that the means of the two distributions are much closer together. As before, if bungling police officers arrest an innocent suspect there's a small chance that the wrong person will be convicted. However, there is now also a significant chance that a guilty person will be set free. This is represented by the yellow/green area under the curve on the left and is a type II error.

    figure 5. The effects of increasing sample size or in other words, number of independent witnesses.

If the standard of judgment is moved to the left by making it less strict the number of type II errors or criminals going free will be reduced. This change in the standard of judgment could be accomplished by throwing out the reasonable doubt standard and instructing the jury to find the defendant guilty if they simply think it's possible that she did the crime. However, such a change would make the type I errors unacceptably high. While fixing the justice system by moving the standard of judgment has great appeal, in the end there's no free lunch.

Fortunately, it's possible to reduce type I and II errors without adjusting the standard of judgment. Juries tend to average the  testimony of witnesses. In other words, a highly credible witness for the accused will counteract a highly credible witness against the accused. So, although at some point there is a diminishing return, increasing the number of witnesses (assuming they are independent of each other) tends to give a better picture of innocence or guilt.

Increasing sample size is an obvious way to reduce both types of errors for either the justice system or a hypothesis test. As shown in figure 5 an increase of sample size narrows the distribution. Why? Because the distribution represents the average of the entire sample instead of just a single data point.

In hypothesis testing the sample size is increased by collecting more data. In the justice system it's increase by finding more witnesses. Obviously, there are practical limitations to sample size. In the justice system witnesses are also often not independent and may end up influencing each other's testimony--a situation similar to reducing sample size. Giving both the accused and the prosecution access to lawyers helps make sure that no significant witness goes unheard, but again, the system is not perfect.

About the only other way to decrease both the type I and type II errors is to increase the reliability of the data measurements or witnesses. For example the Innocence Project has proposed reforms on how lineups are performed. These include blind administration, meaning that the police officer administering the lineup does not know who the suspect is. That way the officer cannot inadvertently give hints resulting in misidentification.

The value of unbiased, highly trained, top quality police investigators with state of the art equipment should be obvious.  There is no possibility of having a type I error if the police never arrest the wrong person. Of course, modern tools such as DNA testing are very important, but so are properly designed and executed police procedures and professionalism. The famous trial of  O. J. Simpson would have likely ended in a guilty verdict if the Los Angeles Police officers investigating the crime had been beyond reproach.


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Statistical Errors Applet

The applet below can alter both the standard of judgment and distance between means for a statistical hypothesis test. It calculates type I and type II errors when you move the sliders. Like any analysis of this type it assumes that the distribution for the null hypothesis is the same shape as the distribution of the alternative hypothesis. 

Note, that the horizontal axis is set up to indicate how many standard deviations a value is away from the mean. Zero represents the mean for the distribution of the null hypothesis.

When the sample size is one, the normal distributions drawn in the applet represent the population of all data points for the respective condition of Ho correct or Ha correct. When the sample size is increased above one the distributions become sampling distributions which represent the means of all possible samples drawn from the respective population. Standard error is simply the standard deviation of a sampling distribution. Note that this is the same for both sampling distributions

Try adjusting the sample size, standard of judgment (the dashed red line), and position of the distribution for the alternative hypothesis (Ha) and you will develop a "feeling" for how they interact.

Note that a type I error is often called alpha. The type II error is often called beta. The power of the test = ( 100% - beta).

Applet 1. Statistical Errors

Note: to run the above applet you must have Java enabled in your browser and have a Java runtime environment (JRE) installed on you computer. If you have not installed a JRE you can download it for free here.

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