In statistics, the false discovery rate (FDR) is a method of conceptualizing the rate of type I errors in null hypothesis testing when conducting multiple comparisons. FDR-controlling procedures are designed to control the FDR, which is the expected proportion of "discoveries" (rejected null hypotheses) that are false (incorrect rejections of the null). [1] Equivalently, the FDR is the expected ratio of the number of false positive classifications (false discoveries) to the total number of positive classifications (rejections of the null). The total number of rejections of the null include both the number of false positives (FP) and true positives (TP). Simply put, FDR = FP / (FP + TP). FDR-controlling procedures provide less stringent control of Type I errors compared to family-wise error rate (FWER) controlling procedures (such as the Bonferroni correction), which control the probability of at least one Type I error. Thus, FDR-controlling procedures have greater power, at the cost of increased numbers of Type I errors. [2]
The modern widespread use of the FDR is believed to stem from, and be motivated by, the development in technologies that allowed the collection and analysis of a large number of distinct variables in several individuals (e.g., the expression level of each of 10,000 different genes in 100 different persons). [3] By the late 1980s and 1990s, the development of "high-throughput" sciences, such as genomics, allowed for rapid data acquisition. This, coupled with the growth in computing power, made it possible to seamlessly perform a very high number of statistical tests on a given data set. The technology of microarrays was a prototypical example, as it enabled thousands of genes to be tested simultaneously for differential expression between two biological conditions. [4]
As high-throughput technologies became common, technological and/or financial constraints led researchers to collect datasets with relatively small sample sizes (e.g. few individuals being tested) and large numbers of variables being measured per sample (e.g. thousands of gene expression levels). In these datasets, too few of the measured variables showed statistical significance after classic correction for multiple tests with standard multiple comparison procedures. This created a need within many scientific communities to abandon FWER and unadjusted multiple hypothesis testing for other ways to highlight and rank in publications those variables showing marked effects across individuals or treatments that would otherwise be dismissed as non-significant after standard correction for multiple tests. In response to this, a variety of error rates have been proposed—and become commonly used in publications—that are less conservative than FWER in flagging possibly noteworthy observations. The FDR is useful when researchers are looking for "discoveries" that will give them followup work (E.g.: detecting promising genes for followup studies), and are interested in controlling the proportion of "false leads" they are willing to accept.
The FDR concept was formally described by Yoav Benjamini and Yosef Hochberg in 1995 [1] (BH procedure) as a less conservative and arguably more appropriate approach for identifying the important few from the trivial many effects tested. The FDR has been particularly influential, as it was the first alternative to the FWER to gain broad acceptance in many scientific fields (especially in the life sciences, from genetics to biochemistry, oncology and plant sciences). [3] In 2005, the Benjamini and Hochberg paper from 1995 was identified as one of the 25 most-cited statistical papers. [5]
Prior to the 1995 introduction of the FDR concept, various precursor ideas had been considered in the statistics literature. In 1979, Holm proposed the Holm procedure, [6] a stepwise algorithm for controlling the FWER that is at least as powerful as the well-known Bonferroni adjustment. This stepwise algorithm sorts the p-values and sequentially rejects the hypotheses starting from the smallest p-values.
Benjamini (2010) said that the false discovery rate, [3] and the paper Benjamini and Hochberg (1995), had its origins in two papers concerned with multiple testing:
The BH procedure was proven to control the FDR for independent tests in 1995 by Benjamini and Hochberg. [1] In 1986, R. J. Simes offered the same procedure as the "Simes procedure", in order to control the FWER in the weak sense (under the intersection null hypothesis) when the statistics are independent. [10]
Based on definitions below we can define Q as the proportion of false discoveries among the discoveries (rejections of the null hypothesis): where is the number of false discoveries and is the number of true discoveries.
The false discovery rate (FDR) is then simply the following: [1] where is the expected value of . The goal is to keep FDR below a given threshold q. To avoid division by zero, is defined to be 0 when . Formally, . [1]
The following table defines the possible outcomes when testing multiple null hypotheses. Suppose we have a number m of null hypotheses, denoted by: H1, H2, ..., Hm. Using a statistical test, we reject the null hypothesis if the test is declared significant. We do not reject the null hypothesis if the test is non-significant. Summing each type of outcome over all Hi yields the following random variables:
Null hypothesis is true (H0) | Alternative hypothesis is true (HA) | Total | |
---|---|---|---|
Test is declared significant | V | S | R |
Test is declared non-significant | U | T | |
Total | m |
In m hypothesis tests of which are true null hypotheses, R is an observable random variable, and S, T, U, and V are unobservable random variables.
The settings for many procedures is such that we have null hypotheses tested and their corresponding p-values. We list these p-values in ascending order and denote them by . A procedure that goes from a small test-statistic to a large one will be called a step-up procedure. In a similar way, in a "step-down" procedure we move from a large corresponding test statistic to a smaller one.
The Benjamini–Hochberg procedure (BH step-up procedure) controls the FDR at level . [1] It works as follows:
Geometrically, this corresponds to plotting vs. k (on the y and x axes respectively), drawing the line through the origin with slope , and declaring discoveries for all points on the left, up to, and including the last point that is not above the line.
The BH procedure is valid when the m tests are independent, and also in various scenarios of dependence, but is not universally valid. [11] It also satisfies the inequality: If an estimator of is inserted into the BH procedure, it is no longer guaranteed to achieve FDR control at the desired level. [3] Adjustments may be needed in the estimator and several modifications have been proposed. [12] [13] [14] [15]
Note that the mean for these m tests is , the Mean(FDR ) or MFDR, adjusted for m independent or positively correlated tests (see AFDR below). The MFDR expression here is for a single recomputed value of and is not part of the Benjamini and Hochberg method.
The Benjamini–Yekutieli procedure controls the false discovery rate under arbitrary dependence assumptions. [11] This refinement modifies the threshold and finds the largest k such that:
Using MFDR and formulas above, an adjusted MFDR (or AFDR) is the minimum of the mean for m dependent tests, i.e., . Another way to address dependence is by bootstrapping and rerandomization. [4] [16] [17]
In the Storey-Tibshirani procedure, q-values are used for controlling the FDR.
Using a multiplicity procedure that controls the FDR criterion is adaptive and scalable. Meaning that controlling the FDR can be very permissive (if the data justify it), or conservative (acting close to control of FWER for sparse problem) - all depending on the number of hypotheses tested and the level of significance. [3]
The FDR criterion adapts so that the same number of false discoveries (V) will have different implications, depending on the total number of discoveries (R). This contrasts with the family-wise error rate criterion. For example, if inspecting 100 hypotheses (say, 100 genetic mutations or SNPs for association with some phenotype in some population):
The FDR criterion is scalable in that the same proportion of false discoveries out of the total number of discoveries (Q), remains sensible for different number of total discoveries (R). For example:
Controlling the FDR using the linear step-up BH procedure, at level q, has several properties related to the dependency structure between the test statistics of the m null hypotheses that are being corrected for. If the test statistics are:
If all of the null hypotheses are true (), then controlling the FDR at level q guarantees control over the FWER (this is also called "weak control of the FWER"): , simply because the event of rejecting at least one true null hypothesis is exactly the event , and the event is exactly the event (when , by definition). [1] But if there are some true discoveries to be made () then FWER ≥ FDR. In that case there will be room for improving detection power. It also means that any procedure that controls the FWER will also control the FDR.
The average power of the Benjamini-Hochberg procedure can be computed analytically [18]
The discovery of the FDR was preceded and followed by many other types of error rates. These include:
The false coverage rate (FCR) is, in a sense, the FDR analog to the confidence interval. FCR indicates the average rate of false coverage, namely, not covering the true parameters, among the selected intervals. The FCR gives a simultaneous coverage at a level for all of the parameters considered in the problem. Intervals with simultaneous coverage probability 1−q can control the FCR to be bounded by q. There are many FCR procedures such as: Bonferroni-Selected–Bonferroni-Adjusted,[ citation needed ] Adjusted BH-Selected CIs (Benjamini and Yekutieli (2005)), [24] Bayes FCR (Yekutieli (2008)),[ citation needed ] and other Bayes methods. [25]
Connections have been made between the FDR and Bayesian approaches (including empirical Bayes methods), [21] [26] [27] thresholding wavelets coefficients and model selection, [28] [29] [30] [31] [32] and generalizing the confidence interval into the false coverage statement rate (FCR). [24]
In information theory, the Rényi entropy is a quantity that generalizes various notions of entropy, including Hartley entropy, Shannon entropy, collision entropy, and min-entropy. The Rényi entropy is named after Alfréd Rényi, who looked for the most general way to quantify information while preserving additivity for independent events. In the context of fractal dimension estimation, the Rényi entropy forms the basis of the concept of generalized dimensions.
The Wilcoxon signed-rank test is a non-parametric rank test for statistical hypothesis testing used either to test the location of a population based on a sample of data, or to compare the locations of two populations using two matched samples. The one-sample version serves a purpose similar to that of the one-sample Student's t-test. For two matched samples, it is a paired difference test like the paired Student's t-test. The Wilcoxon test is a good alternative to the t-test when the normal distribution of the differences between paired individuals cannot be assumed. Instead, it assumes a weaker hypothesis that the distribution of this difference is symmetric around a central value and it aims to test whether this center value differs significantly from zero. The Wilcoxon test is a more powerful alternative to the sign test because it considers the magnitude of the differences, but it requires this moderately strong assumption of symmetry.
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In statistics, family-wise error rate (FWER) is the probability of making one or more false discoveries, or type I errors when performing multiple hypotheses tests.
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In statistics, Fisher's method, also known as Fisher's combined probability test, is a technique for data fusion or "meta-analysis" (analysis of analyses). It was developed by and named for Ronald Fisher. In its basic form, it is used to combine the results from several independence tests bearing upon the same overall hypothesis (H0).
In statistics, the Bonferroni correction is a method to counteract the multiple comparisons problem.
In statistics, the multiple comparisons, multiplicity or multiple testing problem occurs when one considers a set of statistical inferences simultaneously or estimates a subset of parameters selected based on the observed values.
In statistics, the Holm–Bonferroni method, also called the Holm method or Bonferroni–Holm method, is used to counteract the problem of multiple comparisons. It is intended to control the family-wise error rate (FWER) and offers a simple test uniformly more powerful than the Bonferroni correction. It is named after Sture Holm, who codified the method, and Carlo Emilio Bonferroni.
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In statistics, when performing multiple comparisons, a false positive ratio is the probability of falsely rejecting the null hypothesis for a particular test. The false positive rate is calculated as the ratio between the number of negative events wrongly categorized as positive and the total number of actual negative events.
The Newman–Keuls or Student–Newman–Keuls (SNK)method is a stepwise multiple comparisons procedure used to identify sample means that are significantly different from each other. It was named after Student (1927), D. Newman, and M. Keuls. This procedure is often used as a post-hoc test whenever a significant difference between three or more sample means has been revealed by an analysis of variance (ANOVA). The Newman–Keuls method is similar to Tukey's range test as both procedures use studentized range statistics. Unlike Tukey's range test, the Newman–Keuls method uses different critical values for different pairs of mean comparisons. Thus, the procedure is more likely to reveal significant differences between group means and to commit type I errors by incorrectly rejecting a null hypothesis when it is true. In other words, the Neuman-Keuls procedure is more powerful but less conservative than Tukey's range test.
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In statistical hypothesis testing, the error exponent of a hypothesis testing procedure is the rate at which the probabilities of Type I and Type II decay exponentially with the size of the sample used in the test. For example, if the probability of error of a test decays as , where is the sample size, the error exponent is .
The harmonic mean p-value(HMP) is a statistical technique for addressing the multiple comparisons problem that controls the strong-sense family-wise error rate (this claim has been disputed). It improves on the power of Bonferroni correction by performing combined tests, i.e. by testing whether groups of p-values are statistically significant, like Fisher's method. However, it avoids the restrictive assumption that the p-values are independent, unlike Fisher's method. Consequently, it controls the false positive rate when tests are dependent, at the expense of less power (i.e. a higher false negative rate) when tests are independent. Besides providing an alternative to approaches such as Bonferroni correction that controls the stringent family-wise error rate, it also provides an alternative to the widely-used Benjamini-Hochberg procedure (BH) for controlling the less-stringent false discovery rate. This is because the power of the HMP to detect significant groups of hypotheses is greater than the power of BH to detect significant individual hypotheses.