Ask Power - Recent questions and answers

Answered: Error Message

Wed, 05 Aug 2026 23:56:48 +0000

This seemed to be related to a package update. It should have been fixed. Please let us know if it doesn't.

Answered: An error occured. Please check your input or contact us with the code 373368653a8043b6b945e45f7da0fb1c.

Fri, 29 May 2026 13:00:29 +0000

This should have been fixed. Please try again. Thank you for the feedback!

Answered: Error for output of skewness and kurtosis calculation

Thu, 21 May 2026 13:09:36 +0000

Thank you for your feedback. This seemed to be related to an update of package. We believe it has been fixed. Please try again and let us know if it still doesn't work. Thanks.

Answered: what is An error occured. code fc9b0a8833a17c4efc560d684353e3eb.

Thu, 21 May 2026 13:08:47 +0000

Thank you for your feedback. This seemed to be related to an update of package. We believe it has been fixed. Please try again and let us know if it still doesn't work. Thanks.

Answered: An error occured. Please check your input or contact us with the code c5e82b37570d75c49c6a748c12ddb959.

Thu, 21 May 2026 13:07:21 +0000

Thank you for your feedback. This seemed to be related to an update of package. We believe it has been fixed. Please try again and let us know if it still doesn't work. Thanks.

Answered: Trouble uploading data to effect size calculator

Wed, 25 Feb 2026 02:26:32 +0000

Anna, I am literally going through the exact same thing right now and it’s driving me up the wall. My anxiety is through the roof. Honestly, whenever I get this stressed out and things start breaking, I just have to walk away before I smash my monitor.

I usually end up just zoning out on this Telegram channel. They post the craziest videos - it’s the only thing that actually takes my mind off the stress. If you need a distraction while you wait for a fix, definitely check it out. Stay sane, y'all.

Answered: No such analysis!

Wed, 25 Feb 2026 02:08:08 +0000

Google - do you know about it?)

Answered: clarification of input for moderated mediation model 7

Sat, 20 Sep 2025 01:08:00 +0000

Thank you for the feedback.

1. the coefficients are unstandardized ones.

2. the value needs to decide by the user based on the research questions.

Answered: No calculation for mediation analysis models (moderate mediation)

Sat, 20 Sep 2025 01:06:47 +0000

The analysis was based on Monte Carlo simulation. So it will take a while to run to show the results in general.

State when N refers to N per group or total N for each analysis

Mon, 03 Jun 2024 08:12:41 +0000

It would be very helpful if the output screen could clearly state if N refers to N per group or total N for all groups combined. I haven't tried many tests, but my impression is that this not always consistent across tests - and even if it was, it would still be helpful.

Many thanks for the WebPower package - it is awesome

Answered: Model 15 Power calculation no ouput

Fri, 31 May 2024 05:27:05 +0000

Hi there, thanks for reporting this bug. We've fixed this bug and model 15 should now give an output similar to the other models.

Please, Am I doing right?

Tue, 21 May 2024 20:30:45 +0000

Hi, I'm Young, This is my model specification for determining sample size and power. But, it didn't run this way, Did I input the wrong way?

Thank you.

Answered: How to cite and provide reference for Univariate and multivariate skewness and kurtosis calculation?

Thu, 08 Feb 2024 03:44:19 +0000

Use this: Cain, M. K., Zhang, Z., & Yuan, K. (2017). Univariate and multivariate skewness and kurtosis for measuring nonnormality: Prevalence, influence and estimation. Behavior Research Methods, 49(5), 1716–1735. https://doi.org/10.3758/s13428-016-0814-1

Answered: WebPower removed from CRAN?

Wed, 20 Jul 2022 15:19:33 +0000

This was because of the update of package WebPower depends on. We will update the package in the next couple of days.

At the same time, you can download and install it from the archive of CRAN here: https://cran.r-project.org/src/contrib/Archive/WebPower/WebPower_0.7.tar.gz

Thanks.

Answered: An encountered error when checking the results of the diagram based power analysis

Sat, 28 May 2022 15:57:40 +0000

The error seemed to be because that you didn't put the labels on the paths. To add a label to a path, double click the path or select it and then click the "T" icon.

Answered: calculate sample size

Mon, 16 May 2022 20:03:47 +0000

Please specify your question. Thanks.

Answered: Poisson power analysis

Wed, 09 Feb 2022 16:06:07 +0000

We have fixed the integration issue but it will take some time to push it to CRAN. If you need to use it now, you can use the following. Thanks.

library(WebPower)
source("https://webpower.psychstat.org/R/poisson.R")
wp.poisson(n = 40, exp0 = 0.15, exp1 = 3, alpha = 0.05, power = NULL, family = "normal", parameter = c(0, 1))

Using your example, the output is below

Power for Poisson regression
n power alpha exp0 exp1 beta0 beta1
40 0.9534119 0.05 0.15 3 -1.89712 1.098612
50 0.9825010 0.05 0.15 3 -1.89712 1.098612
60 0.9937320 0.05 0.15 3 -1.89712 1.098612
70 0.9978398 0.05 0.15 3 -1.89712 1.098612
80 0.9992791 0.05 0.15 3 -1.89712 1.098612
90 0.9997659 0.05 0.15 3 -1.89712 1.098612
100 0.9999257 0.05 0.15 3 -1.89712 1.098612
URL: http://psychstat.org/poisson

Answered: Power Curve for multigroup mediation models, non normal data and a continious moderator

Sun, 23 Jan 2022 15:03:49 +0000

Yes, you can generate a power curve for multiple group analysis, but not automatically through the wp.mc.sem.power.curve function. You can calculate the power for each sample size and then plot the curve. Below is the code used in Zhang (2013) for multiple group power analysis.

library(bmem)
ex3model<-"
y ~ start(c(.283, .283))*x + c(c1,c2)*x + start(c(.36, .14))*m +c(b1,b2)*m
m ~ start(c(.721, .721))*x + c(a1,a2)*x
m =~ c(1,1)*m1 + start(c(.8,.8))*m2 + start(c(.8,.8))*m3
x ~~ start(c(.25, .25))*x
y ~~ start(c(.81, .95))*y
m ~~ start(c(.87, .87))*m
m1 ~~ start(c(.36, .36))*m1
m2 ~~ start(c(.36, .36))*m2
m3 ~~ start(c(.36, .36))*m3
"
indirect<-'
med1 := a1*b1
med2 := a2*b2
diffmed := a1*b1 - a2*b2
'
bootstrap<-power.boot(ex3model, indirect, nobs=c(400,200), nrep=2000, nboot=1000, parallel='multicore')
summary(bootstrap)

No. Currently, only continuous data are supported. If x is only used as a predictor, you might ignore it.
We have implemented a few calculators (from the R package pwr2ppl) based on the moderated mediation models discussed by Hayes. However, they are not fully tested yet. See here https://webpower.psychstat.org/wiki/models/index#mediation_analysis

Answered: How can I fix the negative beta value in moderation analysis?

Fri, 07 Jan 2022 02:33:14 +0000

This is not really related to power analysis. But to answer your question, the result showed that TLS was actually a moderator here. The coefficient could be positive, negative or even 0. You don't have to get a positive coefficient to conclude the moderation effect.

Answered: Problems with wp.crt2arm

Mon, 03 Jan 2022 20:30:17 +0000

It worked fine on my computer, see the output. I am using WebPower version 0.5.2.

> wp.crt2arm(f=0.6,n=30,J=NULL, icc=.2,power=.8)
Cluster randomized trials with 2 arms
J n f icc power alpha
21.86552 30 0.6 0.2 0.8 0.05
NOTE: n is the number of subjects per cluster.
URL: http://psychstat.org/crt2arm

Answered: A question on the syntax for power analysis using the "wp.mc.sem.basic" function

Mon, 03 Jan 2022 20:27:47 +0000

You can try adding std.lv = T in the function so that

power.2f <- wp.mc.sem.basic(model = model.2f, indirect = NULL, nobs = 100, nrep = 5000, std.lv=T)

Answered: Power analysis for data-model fit using a Monte-Carlo approach

Mon, 03 Jan 2022 19:26:16 +0000

We may add this in the future but don't have a specific timeline for it. Thanks.

Answered: how to perform sensitivity power analysis for independent samples t-test using our effect size?

Mon, 03 Jan 2022 19:25:02 +0000

The question needs clarification but you can calculate your effect size based on t-test for power analysis.

Answered: how to determinate ng and nm in wp.rmanova

Mon, 03 Jan 2022 19:23:00 +0000

Currently, wp.rmanova does not support the analysis with two within-subject factors. It can conduct analysis for one within-subject factor or one within-subject factor and one between-subject factor.

Answered: Simple mediation of fixed effects for random intercept models

Tue, 25 Aug 2020 12:21:59 +0000

If you can specify your model as a wide format SEM model, it can be done using WebPower. Otherwise, the current multilevel method in WebPower only support univariate mixed-effects models.

Answered: Multinomial logistic regression - Power analysis

Fri, 03 Jul 2020 23:56:37 +0000

There is no direct function for the analysis you needed. But if you are only interested in the main effect, you can use the method for ANOVA with binary data here: https://webpower.psychstat.org/models/means06/.

Answered: Power calculation for model with ordinal outcome variable

Fri, 01 May 2020 16:19:33 +0000

wp.regression is for continuous outcomes. WebPower supports logistic regression with a binary outcome and one predictor with different distributions but does not support regression with a categorical outcome.

Answered: Does wp.regression() alpha parameter specify a one-tailed or two-tailed test?

Fri, 01 May 2020 16:03:42 +0000

It only used two-sided test ~ An F-test is used.

Answered: Effect size in RM ANOVA: f or d?

Fri, 24 Apr 2020 00:22:55 +0000

You are right that the effect sizes are defined differently for d and f in t-test and repeated-measures ANOVA. See Section 8.3 in the manual for the definition of f: https://webpower.psychstat.org/wiki/_media/grant/webpower_manual_book.pdf

Answered: How do i unsubscribe?

Fri, 13 Mar 2020 02:13:32 +0000

If you want to delete your account, please email us at webpower@psychstat.org. We will delete your account as well as any files and information related to your account. Thanks.

Answered: post hoc power analysis

Wed, 19 Feb 2020 01:37:28 +0000

Using a?0.39 and b?0.39 should work. Please include a link to your analysis in the error output if something went wrong.

In general, post-hoc power analysis isn't recommended since it does not add anything new to your sample.

Answered: Please send us the URL to your analysis day=20200207&base=67d262748ad87d3963c776f660248cad so that we can fix it.

Fri, 07 Feb 2020 18:43:21 +0000

Thank you for the question. This is a bug in our program and should have been fixed.

Answered: Sample size for a 3-way moderation and moderated mediation

Wed, 25 Sep 2019 19:17:29 +0000

It might be too late to input on this. But you can simulate X, Z, W, Y first and then calculate the interaction terms.

Answered: Effect Size for 3-arm cluster randomized trial

Wed, 25 Sep 2019 19:15:57 +0000

You can find the formular in https://webpower.psychstat.org/wiki/_media/grant/webpower_manual_book.pdf.

Sample Size for binomial distribution

Tue, 25 Jun 2019 20:44:58 +0000

Scenario:

Sensitivity of HCV test is .995 (result is binary, positive/negative)

Power .9

Precision .01

Prevalence is .01

Alpha .05

How many people do I have to screen at the 95% and then at the 99% CI to establish the sensitivity of 99.5%?

Answered: Effect-size ANOVA equation

Thu, 30 May 2019 17:56:27 +0000

The information can be found in the manual here: https://webpower.psychstat.org/wiki/_media/grant/webpower_manual_book.pdf

Answered: Citing Cohen et al for calculation of effect size in 3 arm CRT

Thu, 30 May 2019 17:54:14 +0000

It was based on

Liu, X. S. (2013). Statistical power analysis for the social and behavioral sciences: Basic and advanced techniques. Routledge.

Representing interaction terms in montecarlo SEM power

Tue, 05 Mar 2019 16:37:41 +0000

Hello,

I've been rethinking my previous question and I think, I would be able to solve that one if I knew how to represent interaction terms between exogenous variables with lavaan based syntax for a sem montecarlo power estimation.

Is there anyway I can do it?

For example when dealing with moderated mediation?

Answered: multilevel modeling power analysis

Wed, 21 Nov 2018 06:58:43 +0000

Great question. Really interesting

Answered: accounting for clustered data

Tue, 31 Jul 2018 12:42:41 +0000

Currently, WebPower does not suppose this.

Answered: multivariate skewness and kurtosis online calculation excel file cant be open

Thu, 17 May 2018 03:24:22 +0000

There was an error with our server and it should have been fixed. But let us know if there is still a problem. Thanks.

Answered: Where can I download the WebPower package for R?

Sat, 21 Apr 2018 17:43:12 +0000

The R package WebPower can now be installed within R.

https://cran.r-project.org/package=WebPower

Answered: How to perform a post hoc test after completed one-way ANOVA with binary data

Sat, 21 Apr 2018 17:42:00 +0000

I assume you meant the post-hoc test of two-groups or contrast analysis. WebPower currently does not support this. But you could use a Monte Carlo method to calculate power for it. For doing it, you could use our R package WebPower: https://cran.r-project.org/package=WebPower

Answered: How to cite WebPower

Fri, 09 Feb 2018 10:29:17 +0000

Removed for violation

Answered: Could you please guide me in calculating skewness and kurtosis (Mardia's multivariate coefficient): load data, etc

Sun, 27 Aug 2017 00:28:36 +0000

See if this helps:

http://nd.psychstat.org/blog/estimate_sample_skewness_and_kurtosis_in_popular_statistical_software

Answered: A link is provided for a pdf of the power curve, even if the pdf is empty.

Thu, 05 May 2016 15:18:28 +0000

Thank you for the feedback. Could you provide an example here so that we can identify the problem? You could either provide the link to your power analysis with problem or let us know the power calculator and the parameters you used.

Answered: But why you equalize S with the standard error (S.E.)?

Mon, 14 Mar 2016 11:55:04 +0000

I assume they were sample standard deviations instead of s. e. s. Otherwise, the variances seemed to be a little bit too different. But you can simply change Tue formulas to reflect they are s. e. s.

Answered: Can I estimate the detectable differences with Web Power?

Fri, 11 Mar 2016 14:56:28 +0000

Let me provide a little more details here.

First, use the two-sample t-test power analysis, one can get the effect size is 0.967 with a power of 0.8 and the sample sizes \(n_1=16\) and \(n_2 = 20\).

Suppose the the common standard deviation of the two population can be estimated using the pooled standard deviation such that

\[ s_p = \sqrt{ \frac{(n_1-1)s_1^2+(n_2-1)s_2^2}{n_1+n_2-2} } = 7.32\]

assuming \(s_1=8.38\) and \(s_2=6.35\). Then the detected difference should be 0.967*7.32 = 7.07.

but in that case, where do I put the mean values of my samples?

Thu, 10 Mar 2016 14:47:48 +0000

Thanks Johnny!....but, If I leave the effect size blank, where do I put the values of my means and the standard deviation?.

How to register to be a user?

Tue, 04 Nov 2014 12:16:08 +0000