Analysis of Covariance.

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ANCOVA model with non-significant variables performs better?

I'm running an ANCOVA test with 2 dichotomus and 1 ordinal variable as fixed factors and 2 continuous variables as covariates. I was advised to remove any non significant factors because with them ...
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21 views

ANCOVA with gain scores as IVs?

I ran an experiment (psychology) with two groups: experimental and control. I want to see whether the intervention changed the way in which participants habitually use some emotion regulation (ER) ...
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1answer
25 views

Two way ANCOVA with slight heteroscedasticity

I am about to perform a 2-way ANCOVA but I reject the null hypothesis in Levene's Test with a p-value of 0.023. See standard deviation and sample sizes below. I googled up and down and found people ...
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17 views

ANCOVA with binary response

I was wondering how I could perform an ANCOVA for a binary outcome variable - I have some factor and a continuous covariate that I want to account for in my model. How do I do that? Is there an ...
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9 views

comparison of Akritas et al's & McKean et al's nonparametric ANCOVA models & script of Akritas' model

I'm currently analysing the results of a pretest-posttest experiment with a categorical independent variable with four levels (three treatment groups and a control group) and a continuous dependent ...
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1answer
15 views

Difference between a random slope/intercept model and an ANCOVA with an interaction?

I attempted to run an ANCOVA with one binary predictor, one continuous outcome, and one continuous covariate. I found that there was heterogeneity of regression slopes and thus I concluded that an ...
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21 views

Selecting proper Linear Model with interaction considered

Data consists of Temp(3lv Factor), Water(num), Fert(num), and Growth(num). Considered model is LM with DV: Growth, IV: Temp with Water and Fert as covariate. Fitted 2-Factor ANCOVA (Full Model) and ...
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15 views

ANCOVA with small group size

I have one independent categorical variable 'A' with five groups (n = 1, n = 2, n = 9, n = 18, n = 20). Then I have a continuous dependent variable 'B' and a continuous covariate 'C'. I want to ...
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2answers
63 views

ANCOVA intercepts - does R center data?

I have 2 fixed effects and 2 continuous predictors. Nothing interacts so I believe I want a model of the form: $$Y_{ij} = \mu + \tau_i + \beta_j + \gamma_1(X_{1ij}-\bar{X_1}) + ...
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21 views

questions about ancova model with both random and fixed effects

I have the following data (I show part of the data below) and would like to consider the ANCOVA model with type (a factor with levels 1 and 2) a fixed effect and batch a random effect nested within ...
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12 views

What's the difference between including a variable as a mediator in a mediation analysis, and including a variable as a covariate in ANCOVA?

I understand that ANCOVA has a continuous outcome variable, and a categorical predictor, and (at least) one continuous covariate. Can this be run as a mediation model instead, and if so what are the ...
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13 views

repeated measures ANCOVA; covariate main effect and post-hoc tests

let's say I run a repeated measures ANCOVA, with valence (negative and neutral) as repeated measure and age as a covariate. what I want to know is how age affects my data. 1)I get a significant ...
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1answer
33 views

How do I statistically test whether two intervention types have significantly different effects in a meta-analysis?

I have the following problem: I am researching the difference in effects (as measured by standarized mean differences (SMD)) of two intervention types. (let's call them intervention 1 and intervention ...
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9 views

Can I use a covariate in a RM-ANOVA design and can the covariate be ordinal?

I'm having some trouble deciding what the best approach is to may data. The following is the setup of the study: Between-subjects factor: Group (2 levels) Within-subjects factors: Time (2 levels), ...
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23 views

Should I include weights in LME?

I have two case studies where I am looking at the influence of a trait (trait A) on mortality (m) of trees and seedlings. Following your comments on ...
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41 views

Anova (missing F statistics), Ancova and mixed modelling on nested dataset

I am performing an analysis in R and wonder how to do it since different methods return very different results. My dataset contains Samples and SubSamples as nested random effects and Type, Dose and ...
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1answer
26 views

ANCOVA - F notation - very simple question

I can't understand the number with the F value ($F_{2, 89} = 0.93, p = 0.09$) in result section from the following paper. Relevant section is quoted below. Can anyone please explain this to me? ...
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1answer
34 views

ANCOVA or not? Non continuous covariate

I have done an experiment with an independent and dependent variable. I repeated the experiment 5 times. Obviously all the repeats will be different from one another, some of them significantly so. ...
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40 views

Confused about whether to use t-tests or ANCOVA or ANOVA

I am investing whether vitamin C supplementation can attenuate creatine kinase increases after a strenuous workout. My participants are divided into two groups, VitC group and a Control (no ...
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18 views

Wild bootstrap in “bordeline" case t-test

I have to compare the mean levels of a continuos variable y (ranging 1-20) subdividing my sample in two groups according to a dichotomous variables (i.e gender). Sample sizes of the two groups are ...
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24 views

Repeated measures ANCOVA

I have a data set with training effects and I measured the outcome variable before, during and after training. This is a repeated measures design and I hope to find an effect of training on the ...
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22 views

Is Wild Bootstrap a good strategy in General Linear Model (ANCOVA) with Assumption Violations (both normal residuals and homoscedasticy)?

I need to perform several GLM's (i.e. ANCOVA’s, with a single continuos dependent variable and several predictors, one dichotomous and some other continuos). I was looking for both a significance on ...
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27 views

Test for ANCOVA validity in R

I have a function Y ~ X + trt; where for each trt (treatment group), I have Y and X which correlate with one another. Basically, I want to use ANCOVA to estimate the model parameters, but I don't ...
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32 views

Sampling intercept and slope pairs from lm in R

Let's say I have a model Y~X+trt, and by running lm and summary functions in R I get these results: ...
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23 views

How can I make an ANCOVA in R (Prior to a Cox ph model)?

I want to make an ANCOVA analysis of the survival rate of flies after being on a bad diet for the first 21 days of their lives compared to those always on a normal diet. I have studied the responses ...
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1answer
44 views

Controlling nuisance covariate in lmer

I want to control for a nuisance covariate in a linear model. Since the covariate interacts significantly with one of the fixed factors, the homogeneity of regression slopes assumption is violated for ...
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28 views

Pairwise comparisons with covariates

I have a simple ANCOVA model such that outcome ~ sex + group with outcome being continuous, ...
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14 views

How should one incorporate multiple forms of information about a single variable into a model?

I sincerely hope this is a question that can be definitively answered! I'll do my best. Let's say I have two treatments, flower removal and defoliation, that are fully crossed in an experiment, and ...
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43 views

Longitudinal pre-post test/follow up

I am an undergrad student trying to get my head around statistical analysis. Can someone help me to make sense of the best way to analyse my data? ANOVA, ANCOVA, t-tests....it is all a foreign ...
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1answer
95 views

Ancova Levene's test significant

I did ANCOVA with Depended: Posttest/ Fixed: Group/ Covariate: pretest Levene's test is statistically significant: .002 Is the assumption of homogeneity of variance violated? If yes , what can i do ...
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17 views

Repeated measures ANCOVA - heterogeneity of slopes

In my 2x2x2 mixed model ANCOVA, the covariate is interacting with only one of the main effects (repeated measures). Would it be OK to interpret the rest of the effects and interactions (which were not ...
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39 views

ANCOVA; CIs for the intercepts of each treatment

I am currently doing an ANCOVA, and I need to calculate CIs for the intercepts of each treatment (categorical) group. So, instead of using lm(y~x+group), which gives me SEs of treatment contrasts, I ...
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20 views

I want to compare two calibrations with R… But cannot find the right answer

I first want to precise that I spent 2 hours searching for an answer, couldn't find something that was answering my question. So basically, I ran two calibration with my diagnostic test, so I ...
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1answer
21 views

interpretation baseline corrected ANCOVA

I hope you can help me with some statistic 'problem', or maybe check is more accurate =]. I have data on an experiment with a 2 by 3 design, with session(first/second) and medication(a/b/c) as ...
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15 views

Is it necessary for covariates in an ANCOVA model to be normally distributed? [duplicate]

I am running an ANCOVA model and am trying to remain true to all the statistical assumptions. I noticed that one of my covariates is not normally distributed, I know that the X and Y variables should ...
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1answer
104 views

Using ANCOVA when the covariate is not normally distributed

I have conducted a repeated-measures ANOVA, but a reviewer suspects that the observed main effect of condition are due to a the difference between hit rates of two conditions (one of them is ...
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23 views

Nonlinear form of ANCOVA?

I would like to compare a control group against a test group using something like ANCOVA. However, the covariates are not simple, as some items may have already been increasing with time, others ...
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51 views

Running 2x2x2 ANCOVA in SPSS

I need to conduct an ANCOVA on my data. While I've taken a number of statistics courses, I have never come across ANCOVAs and I want to get some feedback on the correct way to conduct this analysis. ...
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24 views

how to analyse an interaction in a within-subjects repeated ANCOVA post-hoc?

I have pre and post measures of attention that I am examining in a within-subjects repeated measures design. I wanted to check the effect of verbal intelligence [VIQ] on my attention[pre/post] ...
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1answer
27 views

What can be done when a covariate was influenced by the experimental treatment?

I have a problem regarding my data for an ANCOVA. I conducted an experiment with a 2x2 between subjects design (4 different treatment groups) and covariates. Due to the treatment I was just able to ...
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2answers
194 views

When to create a control group with paired T test

So if I wanted to measure the effect of a drug on a group of people, I think I would use a paired T test based on my research. But what if something caused a change in my group that I did not account ...
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13 views

Multiple Ancova's and adjustments to p-values

I am running about 10 Ancova's and am unsure about whether I adjust the overall p-value and then also the posthoc analyses p values. If the categorical independent factor has 3 or more levels. do I ...
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19 views

Compare multiple linear regression lines

I am comparing three regression lines (three spp). I could compared spp1 to spp 2 and spp3 but I couldn't compare spp2 to spp 3. Can anyone help on this? Thanks ...
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23 views

Repeat measures regress

I have a question about using regression for repeat measures analysis. I have a design in which each participant viewed 4 sets of pictures. 2 sets were positive and 2 were negative. There were 2 ...
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171 views

Report in ANCOVA: descriptive means or estimated marginal means?

I have conducted an analysis of covariance, ANCOVA, followed by Bonferroni post-hoc tests, on SPSS. I want to report the differences between my groups in text (APA style) and in a corresponding bar ...
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2answers
122 views

Between-subjects effect becomes non-signicant after centering covariate. Should I center or not?

I've used a general linear model function to run an ANCOVA, involving a categorical predictor (with two levels), a continuous predictor, and their interaction effect. If I don't center the ...
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27 views

Studying pre- and post-treatment change of mean score of a population

I am doing a research studying if a diabetic drug would improve patient's diabetic control. In this study I would enroll a few hundred patients with diabetes. Diabetic control will be measure by the ...
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31 views

When to plot different slopes for different treatments in ANCOVA

I am running an ANCOVA with a treatment of 3 levels and a continuous co-variate. Using step() in r, the simplest model includes the treatment factor and the co-variate, but not their interaction. To ...
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17 views

error variance in ANCOVA vs ANOVA

If for both the treatment and control groups the correlation between the covariate x and the dependent variable y is substantial but negative, the error variance for ANCOVA as compared to ANOVA (less, ...
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30 views

split plot with continuous sub-plot factor?

Question in a nutshell: In a split-plot design, can factor B, the treatment applied to the subplot, be a continuous variable, and can there only be one block?(I read that blocks in split-plot designs ...