Poll: Accuracy of Test Interpretation

Wrath of the Swarm said:
Except that the relative proportions of true positives and false positives will depend on the specific sample population, so that formula does not give universally applicable results.
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Exactly. QED.
 
Do you even know what QED means, or do you just know it's something people say at the end of arguments?

Accuracy is not absolutely definable unless the chance of false positives is the same as the chance of false negatives. Since I defined the accuracy of the test absolutely, alpha must equal beta.

So it is demonstrated. Twit.
 
Wrath of the Swarm said:
Do you even know what QED means, or do you just know it's something people say at the end of arguments?

Accuracy is not absolutely definable unless the chance of false positives is the same as the chance of false negatives. Since I defined the accuracy of the test absolutely, alpha must equal beta.

So it is demonstrated. Twit.
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That which was oughten to be demonstrated, in the leaden prose of literal translation.

What was necessary to prove was that the formula does not give universally applicable results. You did not define the sample population adequately so you cannot have your unique 'accuracy'.

"Since I defined the accuracy of the test absolutely, alpha must equal beta."

Nope. Alpha=Beta defines a unique accuracy, but not the other way around.

"So it is demonstrated." Nope. A poor translation, not even close to the right verb tense, but let's not move on to the relative merits of our educational systems!
 
No, Monkey-boy. Accuracy is not defined outside of a particular sample population only if alpha is not equal to beta. If alpha equals beta, it's perfectly possible to define the absolute accuracy of the test, since that value will no longer change between different populations.

"So it is demonstrated" is a perfectly adequate definition. In contrast, your argument doesn't even approach coherence.

The person making the basic conceptual error shouldn't brag about the quality of his educational experience. Twit.
 
Wrath of the Swarm said:
No, Monkey-boy.Can we manage this without the tiresomely juvenile namecalling? Accuracy is not defined outside of a particular sample population only if alpha is not equal to beta. If alpha equals beta, it's perfectly possible to define the absolute accuracy of the test, since that value will no longer change between different populations.

But, if you only give the accuracy and do not define the sample populations then alpha and beta are not constrained to single values. You did not define the sample population adequately

"So it is demonstrated" is a perfectly adequate definition.

No it isn't. Go and look up your verb moods and tenses.

In contrast, your argument doesn't even approach coherence.

The person making the basic conceptual error shouldn't brag about the quality of his educational experience. Twit.

You are truly a shining wit with these hilarious insults.
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Please discuss: is a phrase a true Spoonerism if it only works in spoken and not written form.
 
I'll stop the puerile name-calling when you cease being a prat.

If it's possible to specify a general accuracy, alpha and beta are set: they're both equal to the general error, which means they're equal to each other. They can take on no other value without contradicting the already-established property of the test.

So by giving the accuracy, I tell you precisely what the alpha and beta are.

This is only about the fifteenth time that particular point has been made, BSM, and not only by me. How many times do we need to repeat it before you understand?

As long as you deserve the names, I'll keep using them. Twit.
 
" To check, he performed a blood test that is known to be about 99% accurate."

Nuff said. You're not going to get it now after all the explaining and it's probably time to stop feeding the troll.

Just remember your Latin homework.

I'm not sure you got the Spooner reference either. So that's another topic to be covered.


By the way folks, doesn't WotS generally imply he/she/it is American? I've not been looking that closely, but that's been my impression. Prat and twit? American or Briton? Child or adult? My reckoning has been sliding down the age groups and East over the Atlantic. Let's turn the thread to more entertaining idle speculation.
 
No - I'm talking about what I said to you. You're clearly not interested in actually thinking about the argument, and you've shown no signs of comprehending any of the points presented to you, so why continue?

Not very good at interpreting semantics, are you?
 
Wrath of the Swarm said:
No - I'm talking about what I said to you. You're clearly not interested in actually thinking about the argument, and you've shown no signs of comprehending any of the points presented to you, so why continue?

Not very good at interpreting semantics, are you?
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Please try to at least pretend you get the jokes. You may feel free to cut and paste the following text if you think you've spotted something that's meant to be humorous and that way you won't stick out too badly from the rest of the class;

"[sarcasm]Ha, ha, very funny[/sarcasm]"

Of, course, you can use it without the square bracket syntax if you really do get the joke.
 
It would be difficult to respond in that manner, since all of your posts to this thread seem to have been jokes. Not amusing ones, but jokes nevertheless.
 
Wrath of the Swarm said:
Let's say that I went for an annual medical checkup, and the doctor wanted to know if I had a particular disease that affects one out of every thousand people. To check, he performed a blood test that is known to be about 99% accurate. The test results came back positive. The doctor concluded that I have the disease.

How likely is it that the diagnosis is correct?

It's best if you don't sit down to work it out. Just give your honest opinion about what you think is likely. If you happen to know the formula that gives the correct answer, feel free to use it.
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I haven't read all the replies, but on at least the first page, no one mentions the issue of base rates, which is what this question is all about. Hey, I give this lecture every semester in an HR class.



If a test is 99% accurate, then if 100 people WITH the disease took it, there would be:

99 hits
1 miss

And, if 100 people without the disease took it, there would be

99 correct rejections and
1 false alarm.

But, the practical value of a test depends on the base rate (the % of the population that has what's being tested for).

The optimal base rate is .50. With a base rate of .50, the test would indeed be 99% accurate at identifying who has the disease and who doesn't

As the base rate departs from .50-- in either direction-- the test becomes less useful.

The base rate in this question is so small .001 that the test has almost no practical value (unless people testing positive get called back to take it a second time).

In fact the odds that you have the disease, given that you test positive, would be real small.

Consider a population of 100,000 people.

100 people have the disease, and of those, 99 would test positve.

99,000 don't have the disease, but of those 1% or 990 of them would test positive.

So, with 100,000 people, there would be 990 + 99 = 1089 positive test results.

But, the probability of actually having the disease would be only:

99/1089 = .0909

If one were to redo my example, but instead use a base rate of .50 (half have it, half don't) the probability would be equal to the test's accuracy-- 99%!


Incidentally, when I lecture on this, I think a good example is those counterfeit pen detectors. Even if they are 99% accurate, I'd argue they are worthless as the base rate for counterfeit bills in the money supply has to be real small.

So, far more often than not, when the pen says the bill is counterfiet, it will actually be real!

Sorry if someone else already answered this, but I am quite positive the above is accurate (either that, or I've misled 1000's of students!)

B
 
If one is talking about a test, then accuracy is the test's reliability and validity, plus the base rate (if one wants to determine proportions of hits, misses, false alarms, correct rejections).

Accuracy, with regard to psychometrics, is a well established construct. So, i don't think the original question is vague-- there's only 1 way to interpret a test having 99% accuracy (as per my previous post).
 
Pesta, where you been the past few days, man?

What is the relationship between reliability/validity and specificity/sensitivity?

~~ Paul
 
The thread was interesting when it started, now it's just tedious and very repetative. I couldn't be bothered to read the last 4 or 5 pages so please forgive me if this was allready put forward.

In my reading of accuracy and the Dream of a thousand cats WotS should be able to calculate the sensitivity and specitifity used in the latter from the 95,3%.

Thanks Rolfe, I think you taught me a lot.
 
Paul C. Anagnostopoulos said:
Pesta, where you been the past few days, man?

What is the relationship between reliability/validity and specificity/sensitivity?

~~ Paul
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I'll jump in on reliabilility/validity.
In psychololgical testing, reliabilty is the repeatability of test scores measured by the test-retest or intertest correlations. A rubber ruler has low reliability.
Validity is the determination of how well a test measures what it it is supposed to measure. There are various kinds: construct, content and predictive.
Validity has another meaning in the design of experiments. Freedom from confounding variables. Reliability is often linked to generality, which has also been termed external validity,
That's why I have avoided posting here before. Terminology may differ between disciplines,
And I use a different example than the first one here.
"If 10 percent of 1000 students use drug X and the test for the drug has a false positive rate of 10 percent, how many people who have no X in their system will fail the test if all 1000 are given the test?"
 
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