Self diagnosis is extremely inaccurate: Only 5-6% of people who score positive on autism questionnaires are actually autistic.

Okay, so this is something I've noticed/known for awhile, but I realise I've never seen anyone post these numbers before. I've included the studies and my working below, but if that's boring to you then the takeaway is this:

If you've taken the autism questionnaires and gotten a positive result, then there's only a 5-6% chance that you are autistic.

In a group of people, if everyone were to take the questionnaire, the number of people falsely diagnosed would outnumber the number of people correctly diagnosed 19 to 1.

Simply put, about 1 in 31 people have ASD (that's about 3% of the population), and based off of study (1) below, about 80% of autistic people were correctly identified using the RAADS-R, AQ-28 and AQ-10. In the same study, only 50% of people without autism were correctly identified as allistic. What does that mean?

Say you take a group of 10,000 people and you make them all take the test. About 323 of them will be autistic. This means that the results would be:

1. About 4839 people will be correctly not identified as autistic
2. About 4839 people will also be incorrectly identified as autistic
3. 258 people will be correctly identified as autistic
4. 64 autistic people will be missed.

This means that the likelihood of having autism, given you have filled out those questionnaires and gotten back a positive, is 258/(4839+258)=5.06%.

I put a second study (2) with slightly different numbers, and based off of that, using just the AQ, the probability that you are autistic because you got a high score on the AQ is 4.78%. Using the stricter cutoff point, the probability is 6.92%**.

Disclaimer: This is assuming that everyone takes the autism questionnaire, I do think that people with autism are more likely to suspect they have autism than allistic people, but I don't know of any research proving that. Also, it would be very very hard to get those numbers up to the point where self-diagnosis is reliable.

Also: If you think I've done something wrong or have research which would make my numbers more accurate then please do share! I am open to all perspectives :)

Sources:

(1) Sizoo BB, Horwitz EH, Teunisse JP, Kan CC, Vissers C, Forceville E, Van Voorst A, Geurts HM. Predictive validity of self-report questionnaires in the assessment of autism spectrum disorders in adults. Autism. 2015 Oct;19(7):842-9. doi: 10.1177/1362361315589869. Epub 2015 Jun 18. PMID: 26088060.

(2) Bezemer, M.L., Blijd-Hoogewys, E.M.A. & Meek-Heekelaar, M. The Predictive Value of the AQ and the SRS-A in the Diagnosis of ASD in Adults in Clinical Practice. J Autism Dev Disord 51, 2402–2415 (2021). https://doi.org/10.1007/s10803-020-04699-7