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	~~{{Refimprove\|date=March 2012}}~~		Maia is what's written on my birth certificate but doable ! call me anything you like. Kentucky is the place I really most. Debt collecting exactly what I do for an income. My wife doesn't like it during I do but things i really like doing is fishing but I have been taking on new things lately. Go to her website as part of your out more: http://www.chicslimo.com/party-bus.html

	The '''positive and negative predictive values''' ('''PPV''' and '''NPV''' respectively) are the proportions of positive and negative results in [[statistics]] and [[diagnostic test]]s that are [[true positive]] and [[true negative]] results.<ref>{{cite book\|last=Fletcher\|first=Robert H. Fletcher ; Suzanne W.\|title=Clinical epidemiology : the essentials\|year=2005\|publisher=Lippincott Williams & Wilkins\|location=Baltimore, Md.\|isbn=0-7817-5215-9\|pages=45\|edition=4th ed.}}</ref> The PPV and NPV describe the performance of a diagnostic test or other statistical measure. A high result can be interpreted as indicating the accuracy of such a statistic. The PPV and NPV is not intrinsic to the test—it depends also on the [[prevalence]]<ref>{{cite journal \|pmid=8038641 \|year=1994 \|last1=Altman \|first1=DG \|last2=Bland \|first2=JM \|title=Diagnostic tests 2: Predictive values \|volume=309 \|issue=6947 \|pages=102 \|pmc=2540558 \|journal=BMJ \|doi=10.1136/bmj.309.6947.102}}</ref> . The PPV can be derived using [[Bayes' theorem]].

	~~Although sometimes used synonymously, a ''positive predictive value'' generally refers to~~ what ~~is established by control groups, while a [[Pre- and post-test probability\|post-test probability]] refers to a probability for an individual. Still, if the individual~~'s ~~[[pre-test probability]] of the target condition is the same as the prevalence in the control group used to establish the positive predictive value, the two are numerically equal~~.

	~~==Definition==~~
	~~===Positive predictive value ===~~
	~~The positive predictive value (PPV) is defined as~~

	::<math> \text{PPV} = \frac{\text{number of true positives}}{\text{number of true positives}+\text{number of false positives}} = \frac{\text{number of true positives}}{\text{number of positive calls}}</math>

	~~where a "true positive"~~ is the event that the test makes a positive prediction, and the subject has a positive result under the gold standard, and a "false positive" is the event that the test makes a positive prediction, and the subject has a negative result under the gold standard.
	~~<!--~~
	~~If the prevalence, sensitivity, and specificity are known, the positive predictive value can be obtained from the following identity:~~

	~~::<math> \text{PPV} = \frac{(\text{sensitivity}) (\text{prevalence})}{(\text{sensitivity}) (\text{prevalence}) + (1 - \text{specificity}) (1-\text{prevalence})} </math> -->~~

	~~===Negative predictive value ===~~
	~~The negative predictive value is defined as:~~

	~~::<math> \text{NPV} = \frac{\text{number of true negatives}}{\text{number of true negatives}+\text{number of false negatives}} =~~
	~~\frac{\text{number of true negatives}}{\text{number of negative calls}}~~
	~~</math>~~

	where a "true negative" is the event that the test makes a negative prediction, and the subject has a negative result under the gold standard, and a "false negative" is the event that the test makes a negative prediction, and the subject has a positive result under the gold standard.
	~~<!--~~
	~~If the prevalence, sensitivity, and specificity are known, the negative predictive value can be obtained from the following identity:~~

	~~::<math>~~
	~~\text{NPV} = \frac{(\text{specificity}) (1 - \text{prevalence})}{(\text{specificity}) (1 - \text{prevalence}) + (1 - \text{sensitivity}) (\text{prevalence})}. </math>~~
	~~-->~~

	~~===Relationship===~~
	~~Although sometimes used synonymously, a ''negative predictive value'' generally refers to~~ what ~~is established by control groups, while a negative [[Pre- and post-test probability\|post-test probability]] rather refers to a probability~~ for an ~~individual~~. ~~Still, if the individual~~'~~s [[pre-test probability]] of the target condition is the same as the prevalence in the control group used to establish the negative predictive value, then the two are numerically equal.~~

	~~The following diagram illustrates how the ''positive predictive value'', [[negative predictive value]], [[Sensitivity and specificity\|sensitivity, and specificity]] are related.~~

	~~{{DiagnosticTesting_Diagram}}~~

	Note that the positive and negative predictive values can only be estimated using data from a [[cross-sectional study]] or other population-based study in which valid [[prevalence]] estimates may be obtained. In contrast, the sensitivity and specificity can be estimated from [[case-control study\|case-control studies]].

	~~===Worked example===~~
	~~Suppose the [[fecal occult blood]] (FOB) screen test is used in 2030 people to look for bowel cancer:~~

	~~{{DiagnosticTesting_Example}}~~

	~~The small positive predictive value (PPV = 10%) indicates that many of the positive results from this testing procedure are false positives. Thus~~ it ~~will be necessary to follow up any positive result with a more reliable test to obtain a more accurate assessment as to whether cancer~~ is present. Nevertheless, such a test may be useful if it is inexpensive and convenient. The strength of the FOB screen test is instead in its [[negative predictive value]] – which, if negative for an individual, gives us a high confidence that its negative result is true.

	~~== Problems with positive predictive value ==~~

	~~===Other individual factors===~~
	Note that the PPV is not intrinsic to the test—it depends also on the prevalence.<ref>{{cite journal \|pmid=8038641 \|year=1994 \|last1=Altman \|first1=DG \|last2=Bland \|first2=JM \|title=Diagnostic tests 2: Predictive values \|volume=309 \|issue=6947 \|pages=102 \|pmc=2540558 \|journal=BMJ \|doi=10.1136/bmj.309.6947.102}}</ref> Due to the large effect of prevalence upon predictive values, a standardized approach has been proposed, where the PPV is normalized to a prevalence of 50%.<ref>{{cite journal \|doi=10.1002/jmri.22466 \|title=Standardizing predictive values in diagnostic imaging research \|year=2011 \|last1=Heston \|first1=Thomas F. \|journal=Journal of Magnetic Resonance Imaging \|volume=33 \|issue=2 \|pages=505; author reply 506–7 \|pmid=21274995}}</ref> PPV is directly proportional to the prevalence of the disease or condition. In the above example, if the group of people tested had included a higher proportion of people with bowel cancer, then the PPV would probably come out higher and the NPV lower. If everybody in the group had bowel cancer, the PPV would be 100% and the NPV 0%.

	To overcome this problem, NPV and PPV should only be used if the ratio of the number of patients in the disease group and the number of patients in the healthy control group used to establish the NPV and PPV is equivalent to the prevalence of the diseases in the studied population, or, in case two disease groups are compared, if the ratio of the number of patients in disease group 1 and the number of patients in disease group 2 is equivalent to the ratio of the prevalences of the two diseases studied. Otherwise, positive and negative [[Likelihood ratios in diagnostic testing\|likelihood ratios]] are more accurate than NPV and PPV, because likelihood ratios do not depend on ~~prevalence~~.

	~~When an individual being tested has a different [[pre-test probability]] of having a condition than the control groups used~~ to establish the PPV and NPV, the PPV and NPV are generally distinguished from the positive and negative [[post-test probabilities]], with the PPV and NPV referring to the ones established by the control groups, and the post-test probabilities referring to the ones for the tested individual (as ~~estimated, for example, by [[Likelihood ratios in diagnostic testing\|likelihood ratios]]). Preferably, in such cases, a large group~~ of ~~equivalent individuals should be studied, in order to establish separate positive and negative predictive values for use of the test in such individuals.~~

	~~===Different target conditions===~~
	~~PPV is used to indicate the probability that in case of a positive test, that the patient really has the specified disease. However there may be~~ more than one cause for a disease and any single potential cause may not always result in the overt disease seen in a patient. There is potential to mixup related target conditions of PPV and NPV, such as interpreting the PPV or NPV of a test as having a disease, when that PPV or NPV value actually refers only to a predisposition of having that disease.

	An example is the microbiological throat swab used in patients with a sore throat. Usually publications stating PPV of a throat swab are reporting on the probability that this bacteria is present in the throat, rather than that the patient is ill from the bacteria found. If presence of this bacteria always resulted in a sore throat, then the PPV would be very useful. However the bacteria may colonise individuals in a harmless way and never result in infection or disease. Sore throats occurring in these individuals is caused by other agents such as a virus. In this situation the gold standard used in the evaluation study represents only the presence of bacteria (that might be harmless) but not a causal bacterial sore throat illness. It can be proven that this problem will affect positive predictive value far more than negative predictive value. To evaluate diagnostic tests where the gold standard looks only at potential causes of disease, one may use an extension of the predictive value termed the [http://www.~~infovoice~~.se/fou/epv Etiologic Predictive Value].<ref>{{cite journal \|doi=10.1002/sim.1119 \|title=The predictive value of microbiologic diagnostic tests if asymptomatic carriers are present \|year=2002 \|last1=Gunnarsson \|first1=Ronny K. ~~\|last2=Lanke \|first2=Jan \|journal=Statistics in Medicine \|volume=21 \|issue=12 \|pages=1773–85 \|pmid=12111911}}</ref>~~

	~~==See also==~~
	* [[Binary classification]]
	* [[Sensitivity and specificity]]
	* [[False discovery rate]]
	* [[Relevance (information retrieval)]]
	* [[Receiver-operator characteristic]]
	* [[Gain (information retrieval)]]
	* [[Diagnostic odds ratio]]

	~~==References and notes==~~
	~~{{reflist\|2}}~~

	~~[[Category:Biostatistics]]~~
	~~[[Category:Statistical ratios]]~~
	~~[[Category:Categorical data]]~~
	~~[[Category:Summary statistics for contingency tables]]~~

	~~[[de:Beurteilung eines Klassifikators#Positiver und negativer Vorhersagewert]]~~

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