In statistical significance testing, the pvalue is the probability of obtaining a test statistic at least as extreme as the one that was actually observed, assuming that the null hypothesis is true.^{[1]} A researcher will often "reject the null hypothesis" when the pvalue turns out to be less than a certain significance level, often 0.05 or 0.01. Such a result indicates that the observed result would be highly unlikely under the null hypothesis (that is, the observation is highly unlikely to be the result of random chance alone). Many common statistical tests, such as chisquared tests or Student's ttest, produce test statistics which can be interpreted using pvalues.
The pvalue is a key concept in the approach of Ronald Fisher, where he uses it to measure the weight of the data against a specified hypothesis, and as a guideline to ignore data that does not reach a specified significance level. Fisher's approach does not involve any alternative hypothesis, which is instead a feature of the Neyman–Pearson approach.
The pvalue should not be confused with the Type I error rate [false positive rate] α in the Neyman–Pearson approach. Although α is also called a "significance level" and is often 0.05, these two "significance levels" have different meanings. Their parent approaches are incompatible, and the numbers p and α cannot meaningfully be compared. Fundamentally, the pvalue does not in itself support reasoning about the probabilities of hypotheses, nor choosing between different hypotheses–it is simply a measure of how likely the data were to have occurred by chance, assuming the null hypothesis is true.
Statistical hypothesis tests making use of pvalues are commonly used in many fields of science and social sciences, such as economics, psychology,^{[4]} biology, criminal justice and criminology, and sociology.^{[5]}
Depending on which style guide is applied, the "p" is styled either italic or not, capitalized or not, and hyphenated or not (pvalue, p value, Pvalue, P value, pvalue, p value, Pvalue, P value).
Examples
Computing a pvalue requires a null hypothesis, a test statistic (together with deciding if the researcher is performing a onetailed test or a twotailed test), and data. A few simple examples follow, each illustrating a potential pitfall.
 One roll of a pair of dice
Suppose a researcher rolls a pair of dice once and assumes a null hypothesis that the dice are fair. The test statistic is "the sum of the rolled numbers" and is onetailed. The researcher rolls the dice and observes that both dice show 6, yielding a test statistic of 12. The pvalue of this outcome is 1/36, or about 0.028 (the highest test statistic out of 6×6 = 36 possible outcomes). If the researcher assumed a significance level of 0.05, he or she would deem this result significant and would reject the hypothesis that the dice are fair.
In this case, a single roll provides a very weak basis (that is, insufficient data) to draw a meaningful conclusion about the dice. This illustrates the danger with blindly applying pvalue without considering the experiment design.
 Five heads in a row
Suppose a researcher flips a coin five times in a row and assumes a null hypothesis that the coin is fair. The test statistic of "total number of heads" can be onetailed or twotailed: a onetailed test corresponds to seeing if the coin is biased towards heads, while a twotailed test corresponds to seeing if the coin is biased either way. The researcher flips the coin five times and observes heads each time (HHHHH), yielding a test statistic of 5. In a onetailed test, this is the most extreme value out of all possible outcomes, and yields a pvalue of (1/2)^{5} = 1/32 ≈ 0.03. If the researcher assumed a significance level of 0.05, he or she would deem this result to be significant and would reject the hypothesis that the coin is fair. In a twotailed test, a test statistic of zero heads (TTTTT) is just as extreme, and thus the data of HHHHH would yield a pvalue of 2/2^{5} = 1/16 ≈ 0.06, which is not significant at the 0.05 level.
This demonstrates that specifying a direction (on a symmetric test statistic) halves the pvalue (increases the significance) and can mean the difference between data being considered significant or not.
 Sample size dependence
Suppose a researcher flips a coin some arbitrary number of times (n) and assumes a null hypothesis that the coin is fair. The test statistic is the total number of heads. Suppose the researcher observes heads for each flip, yielding a test statistic of n and a pvalue of 2/2^{n}. If the coin was flipped only 5 times, the pvalue would be 2/32 = 0.0625, which is not significant at the 0.05 level. But if the coin was flipped 10 times, the pvalue would be 2/1024 ≈ 0.002, which is significant at the 0.05 level.
In both cases the data suggest that the null hypothesis is false (that is, the coin is not fair somehow), but changing the sample size changes the pvalue and significance level. In the first case the sample size is not large enough to allow the null hypothesis to be rejected at the 0.05 level (in fact, the pvalue never be below 0.05).
This demonstrates that in interpreting pvalues, one must also know the sample size, which complicates the analysis.
 Alternating coin flips
Suppose a researcher flips a coin ten times and assumes a null hypothesis that the coin is fair. The test statistic is the total number of heads and is twotailed. Suppose the researcher observes alternating heads and tails with every flip (HTHTHTHTHT). This yields a test statistic of 5 and a pvalue of 1 (completely unexceptional), as this is exactly the expected number of heads.
Suppose instead that test statistic for this experiment was the "number of alternations" (that is, the number of times when H followed T or T followed H), which is again twotailed. This would yield a test statistic of 9, which is extreme, and has a pvalue of $1/2^8\; =\; 1/256\; \backslash approx\; 0.0039,$. This would be considered extremely significant—well beyond the 0.05 level. These data indicate that, in terms of one test statistic, the data set is extremely unlikely to have occurred by chance, though it does not suggest that the coin is biased towards heads or tails.
By the first test statistic, the data yield a high pvalue, suggesting that the number of heads observed is not unlikely. By the second test statistic, the data yield a low pvalue, suggesting that the pattern of flips observed is very, very unlikely. There is no "alternative hypothesis," so only rejection of the null hypothesis is possible) and such data could have many causes – the data may instead be forged, or the coin flipped by a magician who intentionally alternated outcomes.
This example demonstrates that the pvalue depends completely on the test statistic used, and illustrates that pvalues can only help researchers to reject a null hypothesis, not consider other hypotheses.
 Impossible outcome and very unlikely outcome
Suppose a researcher flips a coin two times and assumes a null hypothesis that the coin is unfair: it has two heads and no tails. The test statistic is the total number of heads (onetailed). The researcher observes one head and one tail (HT), yielding a test statistic of 1 and a pvalue of 0. In this case the data is inconsistent with the hypothesis–for a twoheaded coin, a tail can never come up. In this case the outcome is not simply unlikely in the null hypothesis, but in fact impossible, and the null hypothesis can be definitely rejected as false. In practice such experiments almost never occur, as all data that could be observed would be possible in the null hypothesis (albeit unlikely).
If the null hypothesis were instead that the coin came up heads 99% of the time (otherwise the same setup), the pvalue would instead be $0.0199\; \backslash approx\; 0.02.$ In this case the null hypothesis could not definitely be ruled out – this outcome is unlikely in the null hypothesis, but not impossible – but the null hypothesis would be rejected at the 0.05 level, and in fact at the 0.02 level, since the outcome is less than 2% likely in the null hypothesis.
Coin flipping
As an example of a statistical test, an experiment is performed to determine whether a coin flip is fair (equal chance of landing heads or tails) or unfairly biased (one outcome being more likely than the other).
Suppose that the experimental results show the coin turning up heads 14 times out of 20 total flips. The null hypothesis is that the coin is fair, so the pvalue of this result is the chance of a fair coin landing on heads at least 14 times out of 20 flips. This probability can be computed from binomial coefficients as
 $$
\begin{align}
& \operatorname{Prob}(14\text{ heads}) + \operatorname{Prob}(15\text{ heads}) + \cdots + \operatorname{Prob}(20\text{ heads}) \\
& = \frac{1}{2^{20}} \left[ \binom{20}{14} + \binom{20}{15} + \cdots + \binom{20}{20} \right] = \frac{60,\!460}{1,\!048,\!576} \approx 0.058
\end{align}
This probability is the pvalue, considering only extreme results which favor heads. This is called a onetailed test. However, the deviation can be in either direction, favoring either heads or tails. We may instead calculate the twotailed pvalue, which considers deviations favoring either heads or tails. As the binomial distribution is symmetrical for a fair coin, the twosided pvalue is simply twice the above calculated singlesided pvalue; i.e., the twosided pvalue is 0.115.
In the above example we thus have:
 Null hypothesis (H_{0}): The coin is fair; Prob(heads) = 0.5
 Observation O: 14 heads out of 20 flips; and
 pvalue of observation O given H_{0} = Prob(≥ 14 heads or ≥ 14 tails) = 2*(1Prob(< 14)) = 0.115.
The calculated pvalue exceeds 0.05, so the observation is consistent with the null hypothesis, as it falls within the range of what would happen 95% of the time were the coin in fact fair. Hence, we fail to reject the null hypothesis at the 5% level. Although the coin did not fall evenly, the deviation from expected outcome is small enough to be consistent with chance.
However, had one more head been obtained, the resulting pvalue (twotailed) would have been 0.0414 (4.14%). This time the null hypothesis – that the observed result of 15 heads out of 20 flips can be ascribed to chance alone – is rejected when using a 5% cutoff.
Definition
In brief, the (lefttailed) pvalue is the quantile of the value of the test statistic, with respect to the sampling distribution under the null hypothesis. The righttailed pvalue is one minus the quantile, while the twotailed pvalue is twice whichever of these is smaller. This is elaborated below.
Computing a pvalue requires a null hypothesis, a test statistic (together with deciding if one is doing onetailed test or a twotailed test), and data. The key preparatory computation is computing the cumulative distribution function (CDF) of the sampling distribution of the test statistic under the null hypothesis; this may depend on parameters in the null distribution and the number of samples in the data. The test statistic is then computed for the actual data, and then its quantile computed by inputting it into the CDF. This is then normalized as follows:
 onetailed (left tail): quantile, value of cumulative distribution function (since values close to 0 are extreme);
 onetailed (right tail): one minus quantile, value of complementary cumulative distribution function (since values close to 1 are extreme: 0.95 becomes 0.05);
 twotailed: twice pvalue of onetailed, for whichever side value is on (since values close to 0 or 1 are both extreme: 0.05 and 0.95 both have a pvalue of 0.10, as one adds the tails on both sides).
Even though computing the test statistic on given data may be easy, computing the sampling distribution under the null hypothesis, and then computing its CDF is often a difficult computation. Today this computation is done using statistical software, often via numeric methods (rather than exact formulas), while in the early and mid 20th century, this was instead done via tables of values, and one interpolated or extrapolated pvalues from these discrete values. Rather than using a table of pvalues, Fisher instead inverted the CDF, publishing a list of values of the test statistic for given fixed pvalues; this corresponds to computing the quantile function (inverse CDF).
Interpretation
Hypothesis tests, such as Student's ttest, typically produce test statistics whose sampling distributions under the null hypothesis are known. For instance, in the above coinflipping example, the test statistic is the number of heads produced; this number follows a known binomial distribution if the coin is fair, and so the probability of any particular combination of heads and tails can be computed. To compute a pvalue from the test statistic, one must simply sum (or integrate over) the probabilities of more extreme events occurring. For commonly used statistical tests, test statistics and their corresponding pvalues are often tabulated in textbooks and reference works.
Traditionally, following Fisher, one rejects the null hypothesis if the pvalue is less than or equal to a specified significance level,^{[1]} often 0.05, or more stringent values, such as 0.02 or 0.01. These numbers should not be confused with the Type I error rate α in Neyman–Pearsonstyle statistical hypothesis testing; see misunderstandings, below. A significance level of 0.05 would deem extraordinary any result that is within the most extreme 5% of all possible results under the null hypothesis. In this case a pvalue less than 0.05 would result in the rejection of the null hypothesis at the 5% (significance) level.
History
While the modern use of pvalues was popularized by Fisher in the 1920s, computations of pvalues date back to the 1770s, where they were calculated by PierreSimon Laplace:
In the 1770s Laplace considered the statistics of almost half a million births. The statistics showed an excess of boys compared to girls. He concluded by calculation of a pvalue that the excess was a real, but unexplained, effect.
The pvalue was first formally introduced by Karl Pearson in his Pearson's chisquared test, using the chisquared distribution and notated as capital P. The pvalues for the chisquared distribution (for various values of χ^{2} and degrees of freedom), now notated as P, was calculated in (Elderton 1902), collected in (Pearson 1914, pp. xxxi–xxxiii, 26–28, Table XII) The use of the pvalue in statistics was popularized by Ronald Fisher, and it plays a central role in Fisher's approach to statistics.
In the influential book Statistical Methods for Research Workers (1925), Fisher proposes the level p = 0.05, or a 1 in 20 chance of being exceeded by chance, as a limit for statistical significance, and applies this to a normal distribution (as a twotailed test), thus yielding the rule of two standard deviations (on a normal distribution) for statistical significance – see 68–95–99.7 rule.
He then computes a table of values, similar to Elderton, but, importantly, reverses the roles of χ^{2} and p. That is, rather than computing p for different values of χ^{2} (and degrees of freedom n), he computes values of χ^{2} that yield specified pvalues, specifically 0.99, 0.98, 0.95, 0,90, 0.80, 0.70, 0.50, 0.30, 0.20, 0.10, 0.05, 0.02, and 0.01.^{[11]} This allowed computed values of χ^{2} to be compared against cutoffs, and encouraged the use of pvalues (especially 0.05, 0.02, and 0.01) as cutoffs, instead of computing and reporting pvalues themselves. The same type of tables were then compiled in (Fisher & Yates 1938), which cemented the approach.
As an illustration of the application of pvalues to the design and interpretation of experiments, in his following book The Design of Experiments (1935), Fisher presented the lady tasting tea experiment, which is the archetypal example of the pvalue.
To evaluate a lady's claim that she (Muriel Bristol) could distinguish by taste how tea is prepared (first adding the milk to the cup, then the tea, or first tea, then milk), she was sequentially presented with 8 cups: 4 prepared one way, 4 prepared the other, and asked to determine the preparation of each cup (knowing that there were 4 of each). In this case the null hypothesis was that she had no special ability, the test was Fisher's exact test, and the pvalue was $1/\backslash binom\{8\}\{4\}\; =\; 1/70\; \backslash approx\; 0.014,$ so Fisher was willing to reject the null hypothesis (consider the outcome highly unlikely to be due to chance) if all were classified correctly. (In the actual experiment, Bristol correctly classified all 8 cups.)
Fisher reiterated the p = 0.05 threshold and explained its rationale, stating:
It is usual and convenient for experimenters to take 5 per cent as a standard level of significance, in the sense that they are prepared to ignore all results which fail to reach this standard, and, by this means, to eliminate from further discussion the greater part of the fluctuations which chance causes have introduced into their experimental results.
He also applies this threshold to the design of experiments, noting that had only 6 cups been presented (3 of each), a perfect classification would have only yielded a pvalue of $1/\backslash binom\{6\}\{3\}\; =\; 1/20\; =\; 0.05,$ which would not have met this level of significance. Fisher also underlined the frequentist interpretation of p, as the longrun proportion of values at least as extreme as the data, assuming the null hypothesis is true.
In later editions, Fisher explicitly contrasted the use of the pvalue for statistical inference in science with the Neyman–Pearson method, which he terms "Acceptance Procedures".^{[14]} Fisher emphasizes that while fixed levels such as 5%, 2%, and 1% are convenient, the exact pvalue can be used, and the strength of evidence can and will be revised with further experimentation. In contrast, decision procedures require a clearcut decision, yielding an irreversible action, and the procedure is based on costs of error, which he argues are inapplicable to scientific research.
Misunderstandings
Despite the ubiquity of pvalue tests, this particular test for statistical significance has been criticized for its inherent shortcomings and the potential for misinterpretation.
The data obtained by comparing the pvalue to a significance level will yield one of two results: either the null hypothesis is rejected, or the null hypothesis cannot be rejected at that significance level (which however does not imply that the null hypothesis is true). In Fisher's formulation, there is a disjunction: a low pvalue means either that the null hypothesis is true and a highly improbable event has occurred, or that the null hypothesis is false.
However, people interpret the pvalue in many incorrect ways, and try to draw other conclusions from pvalues, which do not follow.
The pvalue does not in itself allow reasoning about the probabilities of hypotheses; this requires multiple hypotheses or a range of hypotheses, with a prior distribution of likelihoods between them, as in Bayesian statistics, in which case one uses a likelihood function for all possible values of the prior, instead of the pvalue for a single null hypothesis.
The pvalue refers only to a single hypothesis, called the null hypothesis, and does not make reference to or allow conclusions about any other hypotheses, such as the alternative hypothesis in Neyman–Pearson statistical hypothesis testing. In that approach one instead has a decision function between two alternatives, often based on a test statistic, and one computes the rate of Type I and type II errors as α and β. However, the pvalue of a test statistic cannot be directly compared to these error rates α and β – instead it is fed into a decision function.
There are several common misunderstandings about pvalues.^{[15]}^{[16]}
 The pvalue is not the probability that the null hypothesis is true, nor is it the probability that the alternative hypothesis is false – it is not connected to either of these. In fact, frequentist statistics does not, and cannot, attach probabilities to hypotheses. Comparison of Bayesian and classical approaches shows that a pvalue can be very close to zero while the posterior probability of the null is very close to unity (if there is no alternative hypothesis with a large enough a priori probability and which would explain the results more easily). This is Lindley's paradox. But there are also a priori probability distributions where the posterior probability and the pvalue have similar or equal values.^{[17]}
 The pvalue is not the probability that a finding is "merely a fluke." As calculating the pvalue is based on the assumption that every finding is a fluke (that is, the product of chance alone), it cannot be used to gauge the probability of a finding being true. The pvalue is the chance of obtaining the findings if the null hypothesis is true.
 The pvalue is not the probability of falsely rejecting the null hypothesis. This error is a version of the socalled prosecutor's fallacy.
 The pvalue is not the probability that replicating the experiment would yield the same conclusion. Quantifying the replicability of an experiment was attempted through the concept of prep.
 The significance level, such as 0.05, is not determined by the pvalue. Rather, the significance level is decided by the person conducting the experiment (with the value 0.05 widely used by the scientific community) before the data are viewed, and is compared against the calculated pvalue after the test has been performed. (However, reporting a pvalue is more useful than simply saying that the results were or were not significant at a given level, and allows readers to decide for themselves whether to consider the results significant.)
 The pvalue does not indicate the size or importance of the observed effect. The two do vary together however–the larger the effect, the smaller sample size will be required to get a significant pvalue (see effect size).
Criticisms
Critics of pvalues point out that the criterion used to decide "statistical significance" is based on an arbitrary choice of level (often set at 0.05).^{[18]} If significance testing is applied to hypotheses that are known to be false in advance, a nonsignificant result will simply reflect an insufficient sample size; a pvalue depends only on the information obtained from a given experiment.
The pvalue is incompatible with the likelihood principle, and pvalue depends on the experiment design, or equivalently on the test statistic in question. That is, the definition of "more extreme" data depends on the sampling methodology adopted by the investigator;^{[19]} for example, the situation in which the investigator flips the coin 100 times yielding 50 heads has a set of extreme data that is different from the situation in which the investigator continues to flip the coin until 50 heads are achieved yielding 100 flips.^{[20]} This is to be expected, as the experiments are different experiments, and the sample spaces and the probability distributions for the outcomes are different even though the observed data (50 heads out of 100 flips) are the same for the two experiments.
Some regard the pvalue as the main result of statistical significance testing, rather than the acceptance or rejection of the null hypothesis at a preprescribed significance level. Fisher proposed p as an informal measure of evidence against the null hypothesis. He called on researchers to combine p in the mind with other types of evidence for and against that hypothesis, such as the a priori plausibility of the hypothesis and the relative strengths of results from previous studies.
Many misunderstandings concerning p arise because statistics classes and instructional materials ignore or at least do not emphasize the role of prior evidence in interpreting p. A renewed emphasis on prior evidence could encourage researchers to place p in the proper context, evaluating a hypothesis by weighing p together with all the other evidence about the hypothesis.^{[1]}
Related quantities
A closely related concept is the Evalue,^{[22]} which is the average number of times in multiple testing that one expects to obtain a test statistic at least as extreme as the one that was actually observed, assuming that the null hypothesis is true. The Evalue is the product of the number of tests and the pvalue.
The 'inflated' (or adjusted) pvalue,^{[23]} is when a group of pvalues are changed according to some multiple comparisons procedure so that each of the adjusted pvalues can now be compared to the same threshold level of significance (α), while keeping the type I error controlled. The control is in the sense that the specific procedures controls it, it might be controlling the familywise error rate, the false discovery rate, or some other error rate.
See also
Notes
References
 expand by hand
 expand by hand





 expand by hand
 expand by hand
 expand by hand

Further reading
 12 Misconceptions, good overview given in following Article
 value
External links
 values calculators for various specific tests (chisquare, Fisher's Ftest, etc.).
 values, including a Java applet that illustrates how the numerical values of pvalues can give quite misleading impressions about the truth or falsity of the hypothesis under test.
ja:有意#p値
This article was sourced from Creative Commons AttributionShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, EGovernment Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a nonprofit organization.