ARTool ARTool
Align-and-rank data for nonparametric factorial ANOVA

Jacob O. Wobbrock, University of Washington [contact]
Lisa A. Elkin, University of Washington
James J. Higgins, Kansas State University
Leah Findlater, University of Washington
Darren Gergle, Northwestern University
Matthew Kay, Northwestern University

Built the Windows version of ARTool
Built the [R] version of ARTool


Current Version 2.2.2

Windows executable:
C# Source code:
[R] version: ARTool package

The Windows version of ARTool requires the Microsoft .NET 4.8.1 runtime.
This software is distributed under the New BSD License agreement.

Our Publications

Please cite the first paper when using ARTool to examine main effects and interactions. Please cite the second paper when using ARTool to conduct ART-C post hoc pairwise comparisons. We also include references to two [R] vignettes for further reading.

Coursera Video

The Aligned Rank Transform (ART) is covered in Prof. Wobbrock's Coursera course on practical statistics. See the course video that covers this procedure.

Purpose of the Aligned Rank Transform (ART)

Classic nonparametric statistical tests—the Kruskal-Wallis test, Mann-Whitney U test, Friedman test, or Wilcoxon signed-rank test—all are one-way tests, permitting the analysis of only a single factor at a time. As a result, multi-factor designs cannot be analyzed with these straightforward rank-based tests. Furthermore, the simple rank transform procedure (RT) of Conover and Iman (1981), which uses an ANOVA on midranks, is reasonable for main effects but causes inflated Type I errors for interaction effects (Salter & Fawcett 1993; Higgins & Tashtoush 1994). Thus, something besides a simple RT procedure is necessary if we are to analyze multiple factors nonparametrically.

But wait—isn't there a nonparametric equivalent to the factorial ANOVA? Surely there must be! Surprisingly, there is no real consensus on such an analysis. Although there has been work by researchers on nonparametric factorial analyses, the resultant methods remain relatively obscure. For a review of some methods, see, e.g., Sawilowsky (1990).

To illustrate the point, consider this table of analyses from UCLA; you will see that no entry is given for two or more independent variables with dependent groups (i.e., repeated measures). A parametric analysis would be, of course, the repeated measures ANOVA, but an equivalent nonparametric analysis is not provided.

The Aligned Rank Transform (ART) procedure was devised to fill this gap (Fawcett & Salter 1984; Higgins et al. 1990; Higgins & Tashtoush 1994; Salter & Fawcett 1985; Salter & Fawcett 1995). For each possible main effect or interaction, all responses (Y) are first "aligned" (Hodges & Lehmann 1962), a process that strips from Y all effects but the one for which the alignment is being carried out. Let an aligned response be Yaligned. The aligned response Yaligned is then assigned midranks; let this ranked response be Yart.

Then a factorial ANOVA can be run on Yart, the aligned-and-ranked responses; vitally, unlike when using the regular rank transform (RT), using an aligned rank transform (ART) ensures that main effects and interactions have appropriate Type I error rates and suitable power.

So why make and publish ARTool (Wobbrock et al. 2011)? ARTool creates, for each possible main effect or interaction, one new aligned column (Yaligned) and one new ranked column (Yart). (Doing so by hand would be extremely tedious and error-prone.) In general, for N factors, ARTool produces 2N-1 aligned columns and 2N-1 ranked columns. You can then analyze the Yart responses with your favorite statistics software.

Importantly, when a facorial ANOVA is run on any given Yart response, only the effect for which Y was aligned and ranked can be extracted from the ANOVA table. The other table entries are meaningless. Thus, for each main effect or interaction in the statistical model, a separate full-factorial ANOVA must be run using a different Yart response.

Example: You have a continuous response Y and two factors, X1 and X2. Using ARTool, you create three aligned responses, Yaligned for X1, Yaligned for X2, and Yaligned for X1×X2. Then these three columns are each ranked by ARTool, resulting in three additional columns: Yart for X1, Yart for X2, and Yart for X1×X2. Then you run three separate full-factorial ANOVAs. The first ANOVA has Yart for X1 as the response, so in the ANOVA table, you only regard the main effect of X1. The second ANOVA has Yart for X2 as the response, so in the ANOVA table, you only regard the main effect of X2. The third ANOVA has Yart for X1×X2 as the response, so in the ANOVA table, you only regard the X1×X2 interaction. In all three ANOVAs, the model factors are X1, X2, and X1×X2.

In our most recent work, we have extended the ART procedure with an additional procedure we refer to as ART-C (Elkin et al. 2021). ART-C is an additional align-and-rank procedure to facilitate post hoc pairwise comparisons. As it turns out, you cannot use regular Yart responses for post hoc pairwise comparisons without inflating Type I error rates. The ART-C procedure allows you to indicate which factor(s) have levels that you would like to compare directly, and ARTool creates additional aligned-and-ranked responses precisely for those comparisons.


Example: You have a continuous response Y and three factors, X1, X2, and X3. Each of these factors has two levels, with X1={a, b}, X2={c, d}, and X3={e, f}. So, your study employs a 2×2×2 factorial design. Using ARTool, you discover a significant X1×X2 interaction. You also discover that X3 is non-significant, either alone or in any interactions. Therefore, you wish to compare levels from X1 and X2 (e.g., conditions {a,c} vs. {b,d}). To do this, you use ARTool's new contrasts feature and indicate X1 and X2 as the contrast factors. ARTool produces a second data table for you with two factors. The first factor we'll call X12, a single "concatenated factor" whose levels are concatenated from those of X1 and X2: {ac, ad, bc, bd}. The second factor is X3, unchanged from its original levels {e, f}, because it was not indicated as a contrast factor. The new response column is Yart-c for X12. Now, in your favorite statistics software, you run a full-factorial model on Yart-c for X12 with factors X12, X3, and X12×X3. You ignore the omnibus test results but, within this model, you request the post hoc pairwise comparisons you desire within X12. For example, to compare {a, c} vs. {b, d}, you compare X12's levels "ac" and "bd". This test will have appropriate Type I errors and power. Of course, if you conduct multiple such comparisons, you should apply a correction, e.g., Holm's sequential Bonferroni procedure (Holm 1979).

Using ARTool.EXE on Windows

Most modern statistics software tools lack built-in features for aligning data. Aligning data is extremely tedious and error-prone to do by hand, especially when more than two factors are involved. That is why we created ARTool.EXE — to do the aligning and ranking for you. This program is downloadable from this page and runs only on Microsoft Windows.

ARTool takes a character-delimited CSV file as input (*.csv). ARTool can work with any text character as a delimiter, or a space or a tab. It can also employ different delimiters for reading in and writing out data tables. The default delimiter is a comma, but European number formats can be handled by telling ARTool to use a delimiter other than a comma (e.g., a semicolon) and to treat commas as decimal points.

The file read in by ARTool must represent a long-format data table (one response Y per row, in the right-most column). The first row must be delimited column names. The first column must be the experimental unit, e.g., Subject. This column, which we'll call S, is not used in ARTool's mathematical calculations, but is useful for clarity in the output table, and is essential anyway for long-format repeated measures tables where the same experimental unit must be listed in multiple rows. As noted, the last column must be the numeric response (Y), i.e., dependent variable. Each column between S and Y represents one factor (X1, X2, X3, etc.) from the experiment. Each possible main effect and interaction results in a new aligned column and a new ranked column in the output CSV file written by ARTool.

The alignment process used is that for a completely randomized design. This can result in reduced power for other designs like split-plots, as described by Higgins et al. (1990). But this is the simplest and most easily generalized alignment algorithm to implement. As it may only reduce power, any significant results should be trustworthy. For more on this issue, see Higgins et al. (1990) and Higgins & Tashtoush (1994).

The output of ARTool for main effects and interactions is a new CSV file, by default with extension *.art.csv. This new file will have, for each effect, an "aligned" column showing the aligned data (Yaligned), and an ART column (Yart) showing midranks applied to the corresponding aligned column. As the original table's columns are also retained, the output data table will have, for N factors, (2+N) + 2*(2N-1) columns. Thus, if the original table has 2 factors, the output table will have (2+2) + 2*(22 - 1) = 10 columns. If the original table has 3 factors, the output table will have (2+3) + 2*(23 - 1) = 19 columns.

The output of ARTool for optional ART-C contrast tests is another CSV file, by default with extension *.art-c.csv. It will contain a new "concatenated factor" combining all of the requested contrast factors. It will also retain, unchanged, any other factors from the original data table not requested for contrasts. It will have one aligned column (Yaligned for ...) and one ranked column (Yart-c for ...).

ARTool performs various checks to ensure its alignment and ranking procedures execute correctly. One important check ensures that each aligned column sums to zero. Users of ARTool can perform a further sanity check by running a full-factorial ANOVA with each aligned column as a response. All effects other than the one for which the column was aligned should be close to F=0.00 and p=1.00.

The CSV files written by ARTool can be opened directly in Microsoft Excel. Statistics software can also open CSV files. There, you can run your ANOVA analyses on the aligned-and-ranked columns.

Using ARTool in [R]

If you use the [R] version of ARTool, things are somewhat streamlined. The art() function performs all of the alignment and ranking that Windows ARTool does, but also runs multiple ANOVAs for you behind the scenes — extracting the appropriate result from each ANOVA — and assembling one final ANOVA table for you. Similarly, the art.con() function, which runs the ART-C procedure, also performs the alignment and ranking steps like Windows ARTool does, but additionally runs all requested pairwise comparisons, applying any indicated adjustment (e.g., "tukey", "holm", "bonferroni", "none", etc.). Below, we offer sample code for using ARTool in [R].

The ARTool package is available on CRAN. The source code for the package is available on Github. You can install the latest CRAN version with this command, which you only need to do once:

		# download and install the ARTool package

Using the ARTool package, the ART procedure is run on a long-format data table in CSV format using the following code:

		# load the ARTool library into memory

		# read a data table into variable 'df'
		df <- read.csv("mydata.csv") # assumes file is in working directory

		# assume 'S' is the name of a within-subjects column
		# assume 'X1' is the name of the first factor column
		# assume 'X2' is the name of the second factor column
		# assume 'X3' is the name of the third factor column
		# assume 'Y' is the name of the response column
		# run the ART procedure on 'df'
		m = art(Y ~ X1 * X2 * X3 + (1|S), data=df) # linear mixed-model syntax; see lme4::lmer()

For between-subjects data, where each row has a unique subject identier S, omit the random factor:

		# omit 'S' for between-subjects data
		m = art(Y ~ X1 * X2 * X3, data=df) # linear model syntax; see lm()

Here is example output:

		Analysis of Variance of Aligned Rank Transformed Data

		Table Type: Analysis of Deviance Table (Type III Wald F tests with Kenward-Roger df)
		Model: Mixed Effects (lmer)
		Response: art(Y)

				   F Df Df.res    Pr(>F)
		1 X1       2.5853485  1      6 0.1589811
		2 X2       5.7810718  1     18 0.0271810  *
		3 X3       0.0044244  1     18 0.9477003
		4 X1:X2    9.3027523  1     18 0.0068918 **
		5 X1:X3    1.7069399  1     18 0.2078310
		6 X2:X3    0.7091528  1     18 0.4107749
		7 X1:X2:X3 0.5721147  1     18 0.4592058
		Signif. codes:   0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 

Post hoc pairwise comparisons can be conducted using ART-C. In the output above, factors X1 and X2 show a significant interaction. Let's assume X1 has levels {a, b} and X2 has levels {c, d}. Code to conduct post hoc pairwise comparisons for the four conditions formed by X1×X2 is as follows:

		library(dplyr)   # for %>% pipe
		art.con(m, "X1:X2", adjust="holm") %>%   # run ART-C for X1×X2
		  summary() %>%  # add significance stars to the output
		  mutate(sig. = symnum(p.value, corr=FALSE, na=FALSE,
		                       cutpoints = c(0, .001, .01, .05, .10, 1),
		                       symbols = c("***", "**", "*", ".", " ")))

Here is example output:

		contrast  estimate   SE df t.ratio p.value sig.
		a,c - a,d   -15.00 4.17 18 -3.595  0.0124  *
		a,c - b,c    -4.75 4.17 18 -1.138  0.8097
		a,c - b,d    -2.25 4.17 18 -0.539  1.0000
		a,d - b,c    10.25 4.17 18  2.456  0.0977  .
		a,d - b,d    12.75 4.17 18  3.056  0.0340  *
		b,c - b,d     2.50 4.17 18  0.599  1.0000

		Results are averaged over the levels of: X3
		Degrees-of-freedom method: kenward-roger
		P value adjustment: holm method for 6 tests

Thus, in this example, it seems that {a, c} vs. {a, d} and {a, d} vs. {b, d} exhibit the significant differences within X1×X2.

ART Mathematics

The mathematics for the general ART procedure were worked out by Higgins & Tashtoush (1994). To the best of our knowledge, the literature on the ART does not present a general formulation for N factors; most publications only address two factors. To enable the creation of ARTool, James J. Higgins worked out the mathematics for N factors. The steps below are those that ARTool uses to align and rank data for main effects and interactions. (You'll see why you wouldn't want to do this by hand!)

Step 1: Residuals. For each raw response Y, compute its residual as

residual = Y - cell mean

The cell mean is the mean response Y̅ for that cell, i.e., over all Y's whose levels of their factors (Xi's) match that of the Y response for which we're computing this residual.

Example: The example table below has two factors (X1, X2), each with two levels {a,b} and {x,y}, and one response column (Y), and shows the calculation of cell means:

Subject    X1  X2  Y   cell mean  
s02ay7 (7+16)/2
s04by8 (8+10)/2

Step 2: Estimated Effects. Compute the "estimated effects." This is best illustrated with an example. Let A, B, C, D be factors with levels:

Ai, i = 1...a
Bj, j = 1...b
Ck, k = 1...c
D, ℓ = 1...d.

Let Ai indicate the mean response Y̅i only for rows where factor A is at level i. Let AiBj indicate the mean response Y̅ij only for rows where factor A is at level i and factor B is at level j. Let AiBjCk indicate the mean response Y̅ijk only for rows where factor A is at level i, factor B is at level j, and factor C is at level k. And so on... Let μ be the grand mean of Y̅ over all rows.

Main effects

The estimated effect for factor A with response Yi is

= Ai
- μ.

Two-way effects

The estimated effect for the A×B interaction with response Yij is

= AiBj
- Ai - Bj
+ μ.

Three-way effects

The estimated effect for the A×B×C interaction with response Yijk is

= AiBjCk
- AiBj - AiCk - BjCk
+ Ai + Bj + Ck
- μ.

Four-way effects

The estimated effect for the A×B×C×D interaction with response Yijkℓ is

= AiBjCkD
- AiBjCk - AiBjD - AiCkD - BjCkD
+ AiBj + AiCk + AiD + BjCk + BjD + CkD
- Ai - Bj - Ck - D
+ μ.

N-way effects

The estimated effect for an N-way interaction is

= N way
- Σ(N-1 way)
+ Σ(N-2 way)
- Σ(N-3 way)
+ Σ(N-4 way)
- Σ(N-h way) // if h is odd, or
+ Σ(N-h way) // if h is even
- μ // if N is odd, or
+ μ // if N is even.

Step 3: Alignment. Compute the aligned data point Yaligned corresponding to the raw data point Y for the effect of interest as:

Yaligned = residual + estimated effect,     i.e.,

= result from step (1) + result from step (2).

Step 4: Ranking. Assign midranks to all Yaligned responses for each new aligned column, thereby creating the Yart columns. With midranks, or averaged ranks, "if a value is unique, its averaged rank is the same as its rank. If a value occurs k times, the average rank is computed as the sum of the value's ranks divided by k" (SAS JMP documentation).

As noted above, ARTool computes aligned data columns (for inspection) and midranks for each of these columns (for ANOVA).

Step 5: Full Factorial ANOVA. This step is not performed by Windows ARTool, but is performed by the [R] version of ARTool, which is to conduct full-factorial ANOVAs on the aligned and ranked responses (Yart). Using the same factors (Xi's) as model terms, conduct a separate ANOVA for each main effect or interaction, being careful to interpret the results only for the factor or interaction for which Yart was aligned and ranked. (Note: Again, if you're using the [R] version of ARTool, it does this for you.)

Example: If you have two factors (X1 and X2), and response (Y), you will run three ANOVAs, each using the same input model (X1, X2, X1×X2), but using a different response variable, one for each aligned-and-ranked Y. That is, one ANOVA will use the response for which Y was aligned-and-ranked for X1. The second ANOVA will use the response for which Y was aligned-and-ranked for X2. The third ANOVA will use the response for which Y was aligned-and-ranked for X1×X2. When interpreting the results in each ANOVA's output, only look at the main effect or interaction for which Y was aligned-and-ranked. So you would extract one result from each of three ANOVAs, for three total results.

ART-C: Contrast Tests with ARTool

A procedure for conducting post hoc pairwise comparisons within the ART paradigm, called ART-C, was devised by Lisa Elkin and Jacob O. Wobbrock (Elkin et al. 2021). This procedure and its mathematics were validated through extensive simulation studies to ensure that Type I error rates were not inflated and that statistical power was comparable to or better than known tests (e.g., t-tests). Indeed, the ART-C procedure does not inflate Type I error rates and has similar or better power than parametric alternatives. For more detail on ART-C, see our UIST 2021 paper or our [R] vignette.

Assuming the same nomenclature as above for Ai, Bj, and Ck, the following steps conduct the ART-C procedure. See Elkin et al. (2021) for more formal mathematical notation.

Step 1: Factor Concatenation. Concatenate all factors whose levels are involved in the desired post hoc pairwise comparisons. The result is a new "concatenated factor." For example, if we have three factors A, B, and C, as above, and wish to compare levels of Ai (i=1...a) and Bj (j=1...b), we would create a new factor AB whose levels are the concatenation of A's and B's levels. That is, for any response Y for which A has level i and B has level j, new factor AB has level ij. For example, if factor A has levels {1, 2} and factor B has levels {1, 2}, then concatenated factor AB would have levels {11, 12, 21, 22}.

After concatenation, the factors involved in the concatenation are dropped, and any factors not involved in the concatentation are kept as-is. In our example, factors A and B are dropped and factor C is left unchanged. So, our resulting two factors are AB and C.

Step 2: Alignment. As we did for main effects and interactions above, align all responses Y. In our example, we would align Y for AB, for C, and for AB×C. However, since we are only interested in post hoc pairwise comparisons among the levels of AB, we can drop the alignment columns for C and for AB×C. Thus, we end up with one alignment column, Yaligned for AB.

Step 3: Ranking. As we did for main effects and interactions, assign midranks to all aligned responses (Yaligned for ...). In our example, this means we create ranked column Yart-c for AB.

Step 4: Full Factorial ANOVA. Conduct a full-factorial ANOVA on the ranked response (Yart-c for ...). The results of this omnibus test are ignored, but conducting it establishes the statistical model within which the post hoc pairwise comparisons take place. In our example, we would conduct an ANOVA with Yart-c for AB  as the response, and AB, C, and AB×C as the model factors.

Step 5: Pairwise Comparisons. Within the context of the full factorial ANOVA just conducted, we now carry out post hoc pairwise comparisons. In our example, we could compare levels of ABij for all ij. For example, as stated above, if factor A has levels {1, 2} and factor B has levels {1, 2}, then concatenated factor AB would have levels {11, 12, 21, 22}. To compare, say, {1, 2} vs. {2, 1}, one would contrast AB's levels {12} vs. {21}. As usual, if multiple pairwise comparisons are conducted, a correction for multiple comparisons should be used (e.g., Tukey, Holm, Bonferroni, etc.).

Sample Data

Four example data sets are included in the ARTool\data folder. The first two are from Higgins et al. (1990). The first of these, named Higgins1990-Table1.csv, shows a mock data set with two between-subjects factors named Row and Column. Each factor has 3 levels. Although in Higgins et al. (1990) this table is represented in wide-format, ARTool requires long-format tables, so it has been rebuilt as such.

A second example is in Higgins1990-Table5.csv. This data is from a real study of moisture levels and fertilizer as it affects the dry matter created in peat. It has two factors, Moisture and Fertilizer. Moisture is a between-subjects factor of 3 levels, while Fertilizer is a within-subjects factor of 4 levels. Twelve trays, each containing four pots of peat, were put in a different moisture condition. Each peat pot on a tray was subjected to a seperate fertilizer. Tray is therefore the experimental unit, and each peat pot on each tray is a "trial." The response variable is the amount of dry matter produced in a pot. In agricultural terminology, this is a classic split-plot design, with Moisture as the whole-plot factor and Fertilizer as the subplot factor. It is instructive to compare the layout of Table 5 in Higgins et al. (1990) to the long-format layout in Higgins1990-Table5.csv.

Note that Higgins1990-Table5.csv has been run through Windows ARTool and so there also exists the resulting file. A post hoc contrasts table was also produced as part of this process, resulting in A demonstration of R's art() and art.con() functions and their comparison to manually processing these *.csv files is given in the R code file Higgins.R.

A third example is Higgins-ABC.csv, which is a mock data set with two between-subjects factors, A and B, and a third within-subjects factor, C. A parametric analysis of variance will show that all main effects and the A×B interaction are significant. An analysis of variance on ART data will show that the same significance conclusions are drawn.

A fourth example is Higgins-ABC.csv renamed to Higgins-Error.csv and given an invalid non-numeric response ("nnn") on the third row of data. When analyzed by ARTool, a red-text error is produced. ARTool produces descriptive error messages, identifying where errors occur so they can be inspected and possibly remedied.

References & Further Reading

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  9. Elkin, L.A., Kay, M., Higgins, J. and Wobbrock, J.O. (2021). An aligned rank transform procedure for multifactor contrast tests. Proceedings of the ACM Symposium on User Interface Software and Technology (UIST '21). Virtual Event (October 10-14, 2021). New York: ACM Press, pp. 754-768. DOI
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  19. Kay, M. (2021). Effect sizes with ART. ARTool [R] package vignette. Updated October 12, 2021. Link
  20. Kay, M., Elkin, L.A. and Wobbrock, J.O. (2021). Contrast tests with ART. ARTool [R] package vignette. Updated October 12, 2021. Link
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This work was supported in part by the National Science Foundation under grants IIS-0811884 and IIS-0811063. Any opinions, findings, conclusions or recommendations expressed in this work are those of the authors and do not necessarily reflect those of the National Science Foundation.

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Last updated May 7, 2024.