World Library  
Flag as Inappropriate
Email this Article

Cochran's Q test

Article Id: WHEBN0015918691
Reproduction Date:

Title: Cochran's Q test  
Author: World Heritage Encyclopedia
Language: English
Subject: Cochran's C test, William Gemmell Cochran, Durbin test, Statistical tests, Nonparametric statistics
Collection: Nonparametric Statistics, Statistical Tests
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Cochran's Q test

In statistics, in the analysis of two-way randomized block designs where the response variable can take only two possible outcomes (coded as 0 and 1), Cochran's Q test is a non-parametric statistical test to verify whether k treatments have identical effects.[1][2][3] It is named for William Gemmell Cochran. Cochran's Q test should not be confused with Cochran's C test, which is a variance outlier test. Put in less technical terms, requires that there only be a binary response (success/failure or 1/0) and that there be 2 or more matched groups (groups of the same size). The test assesses whether the proportion of successes is the same between groups. Often used to assess if different observers of the same phenomenon have consistent results amongst themselves (interobserver variability).

Contents

  • Background 1
  • Description 2
  • Critical region 3
  • Assumptions 4
  • Related tests 5
  • References 6

Background

Cochran's Q test assumes that there are k > 2 experimental treatments and that the observations are arranged in b blocks; that is,

Treatment 1 Treatment 2 \cdots Treatment k
Block 1 X11 X12 \cdots X1k
Block 2 X21 X22 \cdots X2k
Block 3 X31 X32 \cdots X3k
\vdots
\vdots
\vdots
\ddots
\vdots
Block b Xb1 Xb2 \cdots Xbk

Description

Cochran's Q test is

H0: The treatments are equally effective.
Ha: There is a difference in effectiveness among treatments.

The Cochran's Q test statistic is

T = k\left(k-1\right)\frac{\sum\limits_{j=1}^k \left(X_{\bullet j} - \frac{N}{k}\right)^2}{\sum\limits_{i=1}^b X_{i\bullet}\left(k-X_{i\bullet}\right)}

where

k is the number of treatments
X• j is the column total for the jth treatment
b is the number of blocks
Xi • is the row total for the ith block
N is the grand total

Critical region

For significance level α, the critical region is

T > \chi^2_{1-\alpha,k-1}

where Χ21 − α,k − 1 is the (1 − α)-quantile of the chi-squared distribution with k − 1 degrees of freedom. The null hypothesis is rejected if the test statistic is in the critical region. If the Cochran test rejects the null hypothesis of equally effective treatments, pairwise multiple comparisons can be made by applying Cochran's Q test on the two treatments of interest..

Assumptions

Cochran's Q test is based on the following assumptions:

  1. A large sample approximation; in particular, it assumes that b is "large".
  2. The blocks were randomly selected from the population of all possible blocks.
  3. The outcomes of the treatments can be coded as binary responses (i.e., a "0" or "1") in a way that is common to all treatments within each block.

Related tests

  • When using this kind of design for a response that is not binary but rather ordinal or continuous, one instead uses the Friedman test or Durbin tests.
  • The case where there are exactly two treatments is equivalent to McNemar's test, which is itself equivalent to a two-tailed sign test.

References

  1. ^
  2. ^
  3. ^ National Institute of Standards and Technology. Cochran Test

 This article incorporates public domain material from websites or documents of the National Institute of Standards and Technology.

This article was sourced from Creative Commons Attribution-ShareAlike 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, E-Government 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 non-profit organization.
 



Copyright © World Library Foundation. All rights reserved. eBooks from World eBook Library are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.