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Statistics Challenge
- Thread starter Zosimus
- Start date
An ELISA test does not commit logical fallacies for the same reason that guns don't murder people.
People commit logical fallacies. People murder people.
An ELISA test is a tool. You can use that tool properly or improperly. If you say, "Your ELISA test was positive, therefore you will certainly die of AIDS" then YOU have committed a logical fallacy. The test is innocent of your logical failings.
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Yes, that's right. An ELISA test cannot (all one word) be used to determine whether (not if) someone is HIV positive (not has HIV).
At HIV & AIDS - Factors Known to Cause False Positive HIV Antibody Test Results we read that any of the following factors could cause a false positive:
The proper response to this test, therefore, is to try to eliminate as many other possibilities as possible. This is what we call "ruling things out." At the very least, the doctors should ask whether the patient had any vaccinations recently, whether the patient has a cold or the flu, whether the patient has leprosy, tuberculosis, or hepatitis, herpes, or malaria.
You see, the proper way for science to proceed is not to try to prove a theory true but to prove alternate possibilities false.
You cannot prove a theory true, but you can prove theories false.
When will you get that through your head?! Science can only advance by ruling possibilities out. Any scientific study that doesn't rule something out is worthless. Any scientific study that supposedly provides evidence for something is pointless.
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"Both cannot and can not are acceptable spellings, but the first is much more usual."--Oxford Dictionaries
?Cannot? or ?can not?? - Oxford Dictionaries (US)
Then why do people use these ELISA kits to test for HIV?
Then someone who tests positive by ELISA and PCR for HIV, they shouldn't concern themselves at all? They shouldn't even consider the argument that they have HIV? Instead, they should just list the illnesses that they probably don't have and not use protection?
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Why do people get ELISA tests? Various reasons. The first time I got an ELISA test was to convince a girl to have sex with me. The second time was because it was a prerequisite for getting married.
What should you do if you get an ELISA test and it is positive? The first thing you should do is realize that it's probably a false positive. If your doctor tries to put you on AZT or any viral therapy, you should 86 him immediately because he doesn't know what he's doing or talking about. If you're here in Peru, you should immediately get a lung x-ray to rule out tuberculosis and, if you've been bitten by a mosquito recently, you should definitely get checked for malaria and dengue. If you're a woman, you should have a pregnancy test. If these things can be ruled out, then you should schedule a second ELISA test 3-months in the future, and you should stop drinking and taking drugs and you should take selenium supplements until the test date. In all likelihood the follow-up test will be negative. If it's positive again, you might give yourself a 60-40 percent chance of really being HIV positive. You should get tests for all the other possibilities (Herpes, Hepatitis) and you should see a nutritionist.
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According to you, every positive result in a scientific test should be treated as a false positive.
That's not what the science says.
"When two tests are combined, the chance of an inaccurate result is less than 0.1 percent."
HIV Testing | AVERT
Not 60-40. 1 to 999.
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sfs
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Don't be ridiculous -- of course it depends on the disease incidence. If the disease incidence is zero, then it doesn't matter how accurate the test is, zero sick people will test positive.
Which you can't determine without knowing the false negative/true positive rate and the disease incidence. With the disease incidence in the relevant population and both the false negative and the true positive rates, you can calculate the probability that a patient who tests positive has the disease. Without any of those three, you can't.
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sfs
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Your claims would be more persuasive if you hadn't previously given overwhelming evidence that you don't have any idea how science works. They'd still be thoroughly wrong, to be sure.
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Simple math -- something you're no good at.
CDC ? HIV in the United States ? Statistics Overview ? Statistics Center ? HIV/AIDS
There are 320.5 million people in the United States. Since 1.2 million people in the US are HIV+ that means 319.3 million are NOT HIV+
Since 14 percent of the 1.2 million people in the United States who are HIV positive don't know that they are, that's 168,000 people who are HIV positive. That means that our total population is 319.3 million + 168,000 -- I will call that 319.5 million people.
Accordingly your a priori chance of being HIV+ is (drumroll) 0.0526 percent.
So if we take 40,000 people, 21 of those people will be HIV positive and unaware of that fact. So the ELISA test is supposedly 99.7 percent effective, meaning that only 3 people out of 1,000 will falsely test positive. Let's run those numbers:
39,979 people who really are not HIV positive all take the ELISA test. 120 of those people falsely test positive.
21 people really are HIV+ and we have no numbers on false negatives, but we'll assume the test is 100% accurate and that means 21 true positives.
That means that after your first positive ELISA test in the US, your a posteriori chance of being HIV positive is 14.9 percent. That's a generous number that doesn't include all the false positive possibilities included in the list above, and it's really just a guess because the false negative numbers are unknown (and assumed to be zero, to give you the benefit of the doubt).
Now assuming that we take 1,000 people who tested positive the first time and retest them all, and they show up positive the second time, we'll find:
149 true positives
25 false positives
So assuming that the test really is as accurate as they say (doubtful) your chance of being HIV+ after two positive ELISA tests is: 85 percent.
Now 85 percent is a disturbing number, but it's a far cry from the 99.9 percent claim that you have made.
Even then, I'm already ahead of the curve because I have recommended selenium supplementation, which has health benefits for HIV positive people.
You see? Decision theory really works.
Top Stories : Selenium Treatment Beneficial in HIV - by Tim Horn
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Those are the numbers from the scientists who know way more than you do.
Yes, let's do. Out of every 1,000 who do test positive, only 3 test negative by PCR which indicates a false positive. This means that if you test positive, there is only a 0.3% chance that you are not positive. That's how the math works. They take the people who test positive, and THEN they test JUST THOSE PEOPLE to see how many of those are false positives. It is such a simple concept, but you get the math SO SO wrong.
Added in later edit:
I did find this paper that screened HIV patients by seropositive and PCR positive HIV status. Obviously, these are two independent assays that look at very different things. In the PCR assay, they are looking for HIV DNA. In the serum based assay they are looking for human antibodies that react to HIV antigens. Zosimus claims that if you test positive by the serum based test, your chances of truly being HIV positive is just 15%. Let's see if that stacks up. If this is true, then only 15% of serum positive blood donors should be PCR positive, that is actually have viral particles in the serum. What are the results?
"All 24 HIV-2 PCR positive samples tested were positive by the peptide assay. Among 30 HIV-1 PCR and immunoblot positive samples, only one (3.3%) showed an absorbance value above the cut off level. The seven dual positive samples by immunoblot (only positive for HIV-1 by PCR) were negative by the HIV-2 peptide ELISA. There was a 100% concordance between HIV-2 PCR and peptide ELISA."
http://www.sciencedirect.com/science/article/pii/S1386653201001706
Not 15%. Nearly 100% were positive by both PCR and ELISA.
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sfs
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By the way, is there some sort of rule that states that anytime you challenge others' statistical competence, you're going to muff the statistics yourself? Kind of like the rule that spelling flames usually include spelling errors?
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PsychoSarah
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I think that you should reconsider your source of VIRUS MYTH .com since it is probably biased in the extreme. And besides, isn't the risk of a false positive better than going undiagnosed? It seems to me it would be more of a problem if these tests failed to detect real disease rather than having false alarms. HIV treatment should be started immediately to increase how effective it is; people should NEVER assume a positive result is likely to be a false positive, would you do the same for a disease that would kill you in a week? Hmm, either I take potentially unnecessary drugs and supplements for a bit, or I decide not to take them hoping for the chance of a false positive and risk infecting other people and also dying faster. What person chooses the latter option?
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