Type One and Type Two Errors…

When testing a hypothesis there are two possibilities: an effect and no effect. Accuracy is the most important element in psychology, therefore we only accept a result when there is 95% confidence in the effect and only 5% chance results could occur if there was no effect. However even if we’re 95% confident, there is still a chance we can get it wrong! Type One Error and Type Two Errors are the two mistakes we can make:

  1. A Type One Error is when we believe that there is an effect but in reality there isn’t one, this is known as the α level. It is when we wrongly accept the experimental hypothesis and reject the null hypothesis, when it is actually true. This is the worst type of error because false conclusions could do lots of damage e.g. incorrect diagnoses.
  2. A Type Two Error is the opposite; it is when we believe there is no effect when in reality there is. This is most common with small test statistics. The maximum acceptable probability of a type two error is 20%, this is called the β level. In summary, a type two error occurs when we wrongly reject the experimental hypothesis, and accept the null hypothesis when it is false. This is the best type of error to make because no damage is done, the only disadvantage is that nothing good comes of it either e.g. new findings.

An illustration of type one and type two errors in the real world is Rosenhan’s on being sane in insane places study (1973). The study aimed to test whether psychiatrists could reliably diagnose whether a person was insane or not. There was two parts to this study:

  • In the first part of the study, pseudopatients arranged an appointment at the hospital and pretended to be insane. These patients were admitted to the psychiatric ward and labelled as insane, this is an example of a type one error because they were being diagnosed incorrectly.
  • In the second part of the study, Nurses were told that pseudopatients would be trying to enter the hospital however this was a lie. Because nurses believed pseudopatients would enter the hospital, they failed to diagnose many patients that were really insane.  A type two error was made because they failed to diagnose individuals.

From both parts of Rosenhan’s study, it is clear that both type one and type two errors can have a big impact in the real world. But is it more damaging to wrongly diagnose an individual or to fail to diagnose an individual? From the study it is clear that type one errors are the most damaging; diagnosing an individual wrongly can cause all sorts of problems such as hospitalisation, administration of medication and labelling, all of which have a profound negative impact on an individual’s life. Failing to diagnose an individual can have negative impacts for the individual and society, but this impact is not as negative as wrongly diagnosing a person is, as the undiagnosed individual can make another appointment for further diagnosis.

In conclusion, when considering Type One and Type Two Errors, I believe that type one errors are the most damaging in both research and real life because they cause harm rather than just failing to do good.


Andy Field Textbook – Discovering Stats Using SPSS – 3rd Edition

Rosenhan, D.L. (1973) On being sane in insane places. Science, 179. 250-58


41 thoughts on “Type One and Type Two Errors…

  1. baw8 says:

    i agree that type one error is more harmful to individuals as no one likes to be blamed for something they have not done. in research it is the same saying an effect has occurred when it has not can cause many problems such as other researcher trying to repeat the experiment hoping to find the same result which is a waste of the researcher time and money if money is involved as they will not find what they want to find. i feel though that type two error is still bad as failing to report an effect when there is one can be harmful.like in your example if a patient is not diagnosed as insane that person could end up with really problem as well as their diagnosis was not identified. i feel that both type one and type two errors are as important as each other.

  2. saspb says:

    Hey, this was a really nicely written blog – very clear and easy to follow. It’s hard to find anything to argue against as such as this isnt particularly a topic of opinion.
    Both Type 1 errors and Type 2 errors are factors that every researcher must take into account when conducting a study and whilst replication can minimise the chances of these errors occuring, this is not always possible : for example in observational methods or of course quasi-experiments, these are awfully difficult to repeat.
    Some statitions are also now adopting a Type 3 errors which is where the null hypothesis was rejected for the wrong reason. In an experiment, a researcher will firstly suggest a hypothesis and perform their research, after analysing the results statistically, the null is rejected.The problem being that there may be some relationship between the variables, but it could be for a different reason than stated in the hypothesis.


  3. psuc3d says:

    Hello, I really enjoyed your blog! I would just like to mention that although I do think that Type I errors are pretty bad, I don’t think that Type II errors simply fail to do something good. To take a real life example, if a jury were making a decision on the guilt/innocence of a person and made a Type II error, they would be letting the criminal go and walk free despite being guilty. In this case i think that Type II errors are worse but in the example you gave, I completely agree that Type I would have been more dangerous. I think that perhaps the seriousness of the error is dependent on the situation. Think link http://intuitor.com/statistics/T1T2Errors.html explains really well the both types of error within the Justice System. Looking forward to your next blog!

  4. psud63 says:

    Really good explanation of the differences between type 1 and type 2 errors. I do agree with you that type 1 errors are more harmful in research and in the real world as they have the worse implications. You could have possibly included how researchers can reduce the chances of a type 1 and type 2 error occurring.
    Type 1 errors can be avoided by reducing the significance level; e.g if a researcher lowers the significance level, from say 0.05 to 0.01, the chance of the results occurring from chance and causing a type 1 error falls from 5% to 1%.
    Type 2 errors are the opposite and can be avoided by increasing the significance level in order to avoid the chance of rejecting a true alternative hypothesis.


  5. fr4nw says:

    I agree that type I errors can be damaging, particularly when applied in real world scenarios such as incorrectly diagnosing somebody and the effect this could have on someone’s life. Therefore I can see how this would relate to areas such as mental health and how individuals are diagnosed using the DSM (IV) manual and so the diagnostic tools in this manual would need to be backed up by research that had little chance of a type I error. However as it is important that we find appropriate treatment for individuals with mental health disorders, so that they do not harm themselves or others, is it not important that we do not dismiss someone who has a mental disorder? This could be possible if research showed a characteristic / behaviour to not be linked to a mental heath disorder when in reality it is. As a result I feel that making sure individuals do not go undiagnosed and that they receive appropriate treatment is just as important and therefore making a type II error is just as damaging as making a type I error.

  6. psud22psych says:

    Very well done blog, I presume the main aim of the blog was to describe and compare type one and type two errors to see which is worse. I agree with the conclusion that a type 1 error is worse especially in the short term and can cause a lot of damage. A way to expand though could’ve been to describe methods on how to avoid committing this cardinal sin 😉 For example , Nosek, Banaji and Greenwald (2002), suggested that especially with regard to internet research rigorous controls should be put in place to remove extraneous variables which reduces the chance of a type 1 or 2 error as you are more likely (but not guranteed) to get more variance in your scores.

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