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EVIDENCE-BASED-HEALTH  January 2012

EVIDENCE-BASED-HEALTH January 2012

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Subject:

Re: Ten commandments for testing - some problems?

From:

Michael Power <[log in to unmask]>

Reply-To:

Michael Power <[log in to unmask]>

Date:

Wed, 11 Jan 2012 11:53:36 +0000

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (311 lines)

ted

sorry for being too obscure - it was very early when I woke up and
wrote the email

KJB is my abbreviation of King James Bible - Catherine Ebenezer used a
more common abbreviation, KJV, for King James Version of the bible
when she explained the conjugation of verbs in the KJV.

Tom Roper had a typo in his explanation of the third person indicative
- hence my reference to absence of lisp, which is so obscure that
probably even he was unable to make the connection.

I trust thou now knowest what pseudoKJB language is. It is when you
get the lisps in the wrong places.

Michael

On 1/11/12, Ted Harding <[log in to unmask]> wrote:
> Michael, That is lovely stuff! However, while I can decipher
> "TopGear-speak", I'm still puzzled about the acronymics of
> "pseudoKJB language". Would you tell us what "KJB" stand for?
>
> With thanks, and keep up the irony!
> Ted.
>
> On 11-Jan-2012 Michael Power wrote:
>> Paul
>>
>> Thanks for your comments, which have provoked some thoughts.
>>
>> For the time being, I would go with Greg's wording because:
>>
>>    1. Your wording is technically right, but practically wrong.
>>
>>    2. Greg's wording may be technically wrong, but it is practically
>> right.
>>
>>    3. I can't find wording that is both technically and practically
>> correct.
>>
>> Let me explain.
>>
>> If you were to mention "Youden index" to an orthopaedic surgeon in a ski
>> resort, even a bright one who knows what a numerator and denominator are,
>> they would roll their eyes and carry on writing out the request form for a
>> routine preoperative chest X-ray for their 20-year old patient with a
>> fractured tibia.
>>
>> However, if you were to quote Greg's 3rd commandment, even your average
>> orthopaedic surgeon who doesn't know the difference between a mean and a
>> median, might wonder if the X-ray was good for anything other than the
>> local health economy.
>>
>> The reason is that orthopaedic surgeons understand TopGear-speak, and
>> related dialects such as pseudoKJB language. (For those of you whose
>> mother
>> tongue is not UK English, TopGear-speak uses heavy irony and gross
>> exaggeration to amuse the lads and ladettes who have nothing better to do
>> than watch Top Gear TV programmes.)
>>
>> TopGear-speak communicates boring facts and trivial statistics about
>> velocity and acceleration to people who have never heard of Newton, let
>> alone his laws of motion
>>
>> Greg's 10 commandments uses pseudoKJB language to communicate boring facts
>> about important diagnostic statistics -  important as "Thou shalt not
>> kill".
>>
>> The problem (which orthopaedic surgeons do not have), is that
>> TopGear-speak
>> and pseudoKJB language confuse the literal minded and humourless, easily
>> annoy the easily annoyed, allow self-promoting politicians to score cheap
>> hypocritical points, and draw lurking internet trolls out of their caves
>> to
>> be unpleasant.
>>
>> TopGear-speak works because people understand quantitative-looking
>> expressions such as "*99.99% of orthopaedic surgeons are Top Gear fans*",
>> not as the ratio 9999/10000, but as the qualitative feeling that almost
>> all
>> orthopaedic surgeons are petrolheads.
>>
>> (If you have followed the argument this far, you can relax, because I have
>> come at last to my main point.)
>>
>> I think that clinicians decide to test, and decide to act on test results,
>> on the basis of qualitative thinking and gut feeling, even though
>> calculation of Youden indices requires only addition and subtraction, and
>> not the third person singular indicative without a lisp.
>>
>> Our challenge is to get the qualitative, gut feeling, approach based as
>> accurately as possible on a sound statistical foundation. To do this we
>> have first to understand concepts such as prevalence, sensitivity,
>> specificity, predictive values, and Youden index. We then should take the
>> next step and see how this can be translated in practical qualitative
>> rules
>> of thumb.
>>
>> Medow and Lucey have a promising approach in their paper "*A qualitative
>> approach to Bayes' theorem
>> <http://www.ncbi.nlm.nih.gov/pubmed/21862499>*".
>>
>> This seems like a great idea. I wonder if it has been tested in practice?
>>
>> Michael
>>
>> PS If you are not an orthopaedic surgeon, you should be aware that I have
>> resisted the temptation to liberally decorate the above text with metadata
>> such as #IronyAlert.
>>
>> If you are an orthopaedic surgeon, what on earth are you doing here???
>>
>>
>> On 1/10/12, Paul Glasziou <[log in to unmask]> wrote:
>>>   Dear All,
>>> This has been a great idea and discussion to watch :-)
>>> However, before spreading further though, its important to get the maths
>>> straight.
>>> A couple of the items that seem to have problems are:
>>> "3. Thou shalt know that if thou dost order a test with a positive
>>> predictive value below 50%, thou mightest as well toss a coin."
>>> That's incorrect for several reasons. A test may be *very* useful even
>>> with a low PPV - if the negative predictive value is good, and that is
>>> what is needed.
>>> For example, the Ottawa Ankle rule is a very sensitive "test" for
>>> fractures*, so negative results rule out (SnNout). But the Rule is not
>>> very specific, so the positive predictive value is generally less than
>> 50%.
>>> That still seems a useful test - saving around 25% of ankle Xrays - but
>>> with weak PPV.
>>> A "coin toss" is indeed a useless test - but that does not mean a PPV
>>> less than 50%, but that the pre-test and posi-test probabilities are the
>>> same. That is the predictive value positive would be the same as the
>>> pre-test probability - which may be above or below 50% depending on
>>> clinical circumstances.
>>> So how can you pick a useless test? Two ways to tell are: if the Youden
>>> Index (= 1 - (sensitivity + specificity) ) is zero or (equivalently) the
>>> (diagnostics) Odds Ratio is 1 (as hence both LR+ and LR- will also be 1).
>>> So 3 might read something like:
>>> "3. Thou shalt know that if thou dost order a test with a Youden Index
>>> near 0, thou mightest as well toss a coin."
>>>
>>> There is a related problem with Commandment 4:
>>> "4 ... Thou shalt recognise that in a low-prevalence population, even a
>>> very sensitive test hath poor predictive value, and if thou shouldest
>>> order such a test, thou mightest as well toss a coin."
>>> Again if we are interested in ruling out, then we might us the sensitive
>>> test (SnNout) in low prevalence populations. For example, using highly
>>> sensitive d-dimer to rule out PE in someone with a some suspcion, but
>>> low probability, of PE.
>>> Incidentally, prevalence matters most when tests are only modestly
>>> accurate. For a perfect test (100% sensitive and specific) prevalence
>>> becomes almost irrelevant. And for either SpPin (100% specific) or
>>> SnNout (100% sensitive), then its of low relevance for the +ve or -ve
>>> results respectively.
>>>
>>> Happy Commandment making ;-)
>>> Paul
>>> * see BMJ. 2003 Feb 22;326(7386):417.
>>>
>>>
>>>
>>> On 1/10/2012 2:00 AM, Ash Paul wrote:
>>>> Dear colleagues,
>>>>
>>>> Here's the corrected draft, with grateful help received from Ms
>>>> Catherine Ebenezer:
>>>>
>>>> 1. Before thou dost order a test, thou shalt remember that “All tests
>>>> can do harm. Many tests do good. Some tests do more good than harm”.
>>>> 2. Thou shalt know the predictive positive value and predictive
>>>> negative value of the test before thou dost order it.
>>>> 3. Thou shalt know that if thou dost order a test with a positive
>>>> predictive value below 50%, thou mightest as well toss a coin.
>>>> 4. Thou shalt know the prevalence of the disease in the population
>>>> from which the person thou art going to test cameth; for combined with
>>>> positive predictive value or negative predictive value, thou canst
>>>> then ensure that thou hast interpreted the result properly. Thou shalt
>>>> recognise that in a low-prevalence population, even a very sensitive
>>>> test hath poor predictive value, and if thou shouldest order such a
>>>> test, thou mightest as well toss a coin.
>>>> 5. Thou shalt know exactly what thou mightest do with the result of
>>>> the test before thou dost order it. “If thou knowest not what thou
>>>> shouldest do with the result, or if the test be not part of an
>>>> evidence-based pathway, then shalt thou DESIST from ordering such a
>>>> test.”
>>>> 6. Thou shalt know the cost of the test thou intendest to order,
>>>> before thou dost order it, and also thou shouldest enquire whether
>>>> there be cheaper ways of finding the same result.
>>>> 7. Thou shalt know whosoever else hath in the past ordered, or might
>>>> in the future order, the same test under the same set of
>>>> circumstances, and then shalt thou ensure that they know that they do
>>>> but waste resources, since thou hast performed the same already
>>>> before. Therefore, thou shalt record the result of the test which thou
>>>> dost order very clearly that all others might see it.
>>>> 8. Thou shalt inform patients that “shinier scanners and swizzier
>>>> kits" might well give better resolution pictures, but might not
>>>> actually change a clinical management decision.
>>>> 9. Thou shalt simply not ask or order patients to undertake diagnostic
>>>> tests for which there be no proven evidence of benefit.
>>>> 10. Thou shalt explain to the patient, clearly and without bias, that
>>>> which the test can and cannot tell thee, and thou shalt ask them if
>>>> they are content to proceed in such wise.
>>>> Regards,
>>>>
>>>> Ash
>>>>
>>>>     ********
>>>>
>>>> ------------------------------------------------------------------------
>>>>     *From:* Evidence based health (EBH)
>>>>     [mailto:[log in to unmask]] *On Behalf Of *Ash
>>>> Paul
>>>>     *Sent:* 09 January 2012 13:22
>>>>     *To:* [log in to unmask]
>>>>     *Subject:* Re: Ten commandments for testing
>>>>     Dear Greg,
>>>>     I have biblified yours/Anna's list and circulated it to our NHS
>>>>     Public Health Commissioners e-group. I have proposed calling them
>>>>     the Fell-Sayburn Commandments
>>>>
>>>>        1. Before thou ordereth a test, thou shalt remembereth that
>>>>           “All tests can doeth harm. Many tests doeth good. Some tests
>>>>           doeth more good than harm”
>>>>
>>>>        2. Thou shalt knoweth the predictive positive value and
>>>>           predictive negative value of the test before thou ordereth it
>>>>
>>>>        3. Thou shalt knoweth that if thou ordereth a test with a
>>>>           positive predictive value below 50%, thou might as well
>>>>           tosseth a coin
>>>>
>>>>        4. Thou shalt knoweth the prevalence of the disease in the
>>>>           population from which the person thou art going to test
>>>>           cometh from; for combined with positive predictive value or
>>>>           negative predictive value, thou can then ensureth that thou
>>>>           hast interpreted the result properly. Thou shalt recogniseth
>>>>           that in a low prevalence population, even a very sensitive
>>>>           test has poor predictive value, and if thou ordereth such a
>>>>           test, thou might as well tosseth a coin.
>>>>
>>>>        5. Thou shalt knoweth exactly what thou art going to doeth with
>>>>           the result of the test before thou ordereth it. “If thou
>>>>           doth not know what to doeth with the result, or if the test
>>>>           is not part of an evidenced-based pathway, then thou shalt
>>>>           DESIST from ordering such a test.”
>>>>
>>>>        6. Thou shalt knoweth the cost of the test thou art going to
>>>>           ordereth, before thou ordereth it, and also enquireth
>>>>           whether there are cheaper ways of finding the same result.
>>>>
>>>>        7. Thou shalt knoweth who else might have in the past, or might
>>>>           in the future, ordereth the same test under the same set of
>>>>           circumstances, and then ensureth they know they are wasting
>>>>           resources as thou hast done it already before. Therefore,
>>>>           thou shalt recordeth the result of the test thou ordereth
>>>>           very clearly for all others to see.
>>>>
>>>>        8. Thou shalt informeth patients that “shinier scanners and
>>>>           swizzier kits" might well giveth better resolution pictures
>>>>           but they might not actually changeth a clinical management
>>>>           decision
>>>>
>>>>        9. Thou shall simply not asketh or ordereth patients to
>>>>           undertake diagnostic tests for which there is well proven
>>>>           evidence of no benefit
>>>>
>>>>        10. Thou  shalt explaineth to the patient, clearly and without
>>>>     bias, what the test can and canst tell thou, and thou shalt asketh
>>>>     them if they are happy to goeth  ahead on that basis.
>>>>     Regards,
>>>>
>>>>     Ash
>>>>
>>>>
>>>> ------------------------------------------------------------------------
>>>>         *From:* Fell Greg <[log in to unmask]>
>>>>         *To:* [log in to unmask]
>>>>         *Sent:* Monday, 9 January 2012, 12:19
>>>>         *Subject:* Re: Ten commandments for testing
>>>>         accept the need for a “full” version (with refs and all
>>>>         that)….guess it depends on intended audience and endpoint. I
>>>>         feel the slightly humerous biblified version might have more
>>>>         “reach” (on account of the humour factor)….but accept that
>>>>         “reach” and “influence” might be two slightly different things.
>>>>         I am not going to die in a ditch about either.
>>>>         The important thing is that the list sees the light of day
>>>>         outside this e group – published / blogged / other. I am
>>>>         intending on using it extensively locally.
>>>>         Do colleagues feel this is publishable.
>>>>         gf
>>>>
>>>
>>>
>>> --
>>> Paul Glasziou
>>> Bond University
>>> Qld, Australia 4229
>>>
>>>
>
> ----------------------------------
> E-Mail: (Ted Harding) <[log in to unmask]>
> Date: 11-Jan-2012
> Time: 10:45:03
>
> This message was sent by XFMail
> ----------------------------------
>

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