Tagged Questions

Analysis of Covariance.

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4
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0answers
73 views

Implementing ANCOVA via residuals

While it is often mis-used, I'd like to provide users of ezANOVA() (from the ez package for R) with the ability to specify covariates for ANCOVA. Since ...
2
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0answers
127 views

Mixed model in R with paired design and covariates

I make habitat suitability models for animal species. The purpose of my research is to investigate the accuracy of different models. I clearly have a nested design: accuracy_measure -> response ...
0
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0answers
131 views

Interpreting repeated measures ANCOVA in R

I am new to R and trying an analysis that I can't figure out how to do in JMP. I incubated wood samples in various stages of decay at four different temperatures and measured respiration rate and ...
1
vote
0answers
28 views

Removing an interaction term after checking for homogeneity of regression slopes [duplicate]

Possible Duplicate: Interpreting main effect and interaction In a GLM, I had one categorical predictor and one covariate. To ensure the covariate was consistent across levels of my ...
2
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0answers
185 views

Effect of mediating variable independent of effect of possible covariate

Here is a rather involved question. It can be summed up as follows: how can I test the effect of a potential mediating variable, independent of the effect of a possible covariate with which it is ...
5
votes
1answer
647 views

Assumptions of generalised linear model

I have made a generalised linear model with a single response variable (continuous/normally distributed) and 4 explanatory variables (3 of which are factors and the fourth is an integer). I have used ...
2
votes
1answer
484 views

Linear model overfitting due to too many covariates

My study design involves a control and 2 test groups plus some covariates. Each group consists of around 20 observations. In total I look at around 1,000 variables. I created a linear model using the ...
0
votes
2answers
761 views

Compare several regression lines ANCOVA?

I´m about to make a comparison I had never performed so I ask you to excuse me if the question is too obvious. I´m trying to analyze the relation between number of movements and prey capture time on ...
7
votes
1answer
268 views

Identifying differences among calibration curves: ANCOVA?

I've got a large number of calibration curves (i.e. instrument response vs. analyte concentration), and I'd like to check whether they are different from each other. Exactly one curve is associated ...
0
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0answers
815 views

Using lmer() with missing levels

I believe I need to run a mixed-effects ANCOVA, and I am having difficulty, presumably, because I have missing data in one of my fixed effects or an incomplete design. I have a random grouping ...
8
votes
1answer
277 views

Is it acceptable to run two linear models on the same data set?

For a linear regression with multiple groups (natural groups defined a priori) is it acceptable to run two different models on the same data set to answer the following two questions? Does each ...
7
votes
1answer
247 views

How to analyse RCT where significant baseline differences exist despite randomisation?

I'm analyzing a double-blind, placebo controlled RCT using an ANCOVA in R, where the predicted "treatment effect" shows up before the treatment has occurred! The goal of the study is to determine if ...
2
votes
1answer
102 views

How to determine if a change in F value when introducing a covariate, is significant?

My question may be very basic but I hope that you can help me. I have got two groups (A and B). I am measuring two continuous biological measures (X and Y) which are significantly positively ...
2
votes
1answer
105 views

Proper order of variables in unbalanced ANOVA

If you have unequal sample sizes in cells, then the order in which you enter model terms changes your results for sequential or Type I SS. The first variable to enter the model is allocated its ...
1
vote
1answer
114 views

Ancova without intercept in R

Here is an exemple of fitting an ANCOVA model without intercept in R: ...
1
vote
1answer
204 views

Help with change scores and/or ANCOVA design

I am carrying out an evaluation for an intervention using secondary data. Unfortunately the design of the study is weak as participants cannot be randomised. I am looking to see whether the ...
1
vote
1answer
109 views

Is it allowed to transform x and y in an ANCOVA?

in my ANCOVA analysis, I have the case that my Y and X show a nonlinear pattern. Transforming either only Y or X does not help to stabilize variance. I did not find any hint that doing an ANCOVA in ...
2
votes
1answer
104 views

How to control for depression when sample was measured using two different tests?

I am looking at pre and post test depression scores in a 2x3 design with treatment and genotype as the IVs and depression score as the DV. However, I have to use one depression test for younger ...
2
votes
1answer
323 views

Effect size of the results of an ANCOVA

I have compared two regression models using ANCOVA by following this tutorial but using my own data. Here is a snippet of my data: ...
5
votes
1answer
1k views

Repeated-measures ANCOVA in R?

experts, I would like to ask for advice regarding the analysis of a dataset I am currently working on. In the experiment subjects were tested in a reaction time task (RT as dependent variable). In ...
2
votes
3answers
558 views

How to get rid of heterogeneity of regression slopes using multilevel modeling?

I have problem with homogeneity of regression slopes in ANCOVA. I have two species of snakes and I want to compare their tail lengths (dependent variable), that are dependent on body length (in ANCOVA ...
1
vote
1answer
2k views

How to interpret non-significant effect of a covariate in ANCOVA?

I need to use ANCOVA to analyse effect of executive functions (EF-covariate) on scores collected during a test. I want to compare this effect between a control and schizophrenics group (variable ...
2
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0answers
385 views

Using variations of ANCOVA for non-normal distributions

My first question here, hope to be relevant to the community. Im into CS course and a total rookie on statistics, so let me know if more detail is needed and I will edit the post. I have a problem ...
1
vote
3answers
3k views

ANCOVA or repeated measures ANOVA?

I have 3 groups randomised to intervention A or B or control. Participants were measured pre and post intervention - should I use repeated measures ANOVA or ANCOVA with the variable measure at time 1 ...
5
votes
1answer
1k views

How to choose between ANOVA and ANCOVA in a designed experiment?

I am conducting an experiment which has the following: DV: Slice consumption (continuous or could be categorical) IV: Healthy message, unhealthy message, no message (control) (3 groups in which ...
0
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3answers
913 views

My groups differ on covariate

In this case I presume loss of ANCOVA power, so I don´t know what type of analysis should I run next. There was significant difference in covariate between groups (p=0,008). Is there some solution? ...
2
votes
1answer
484 views

Difference between ANCOVA and Hierarchical Regression

Is there a difference between ANCOVA (as performed under the 'General Linear Model (GLM)') and Hierarchical Regression (as performed under 'Regression') in SPSS? I am testing the main effects and ...
1
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0answers
120 views

Interaction terms and BUGS

It occurred to me that how one writes an interaction term in BUGS depends on the types of predictors that are "interacting". Suppose data that are long (rather than wide). Then, a model with an ...
1
vote
1answer
298 views

multiple continuous independent variables, single dependent var: which analysis?

I am setting up a linear analysis of a single continuous dependent variable, with multiple (up to 11!) continuous independent variables. I am interested in doing something like an ANCOVA that will ...
5
votes
3answers
829 views

ANCOVA with repeated measures in R

I am a novice to program R and have been trying to perform a repeated measures ANCOVA with Temperature as the dependent variable, Site as the independent variable, Date as the covariate and Year as ...
1
vote
2answers
2k views

Significant interaction between covariate and factor in SPSS GLM

In testing gender difference on the relationship between variable A and B, A is the covariate (or independent variable) B is the dependent variable Gender is the factor As I understand it, if ...
0
votes
2answers
1k views

General linear model with interaction term in SPSS

Following my question here, I will be most grateful for further assistance to clear my confusion. The confusion is because I have read in many places that when you use ANCOVA in GLM (in SPSS), the ...
1
vote
0answers
326 views

Fitting generalized linear model with covariate

In our experimental design, we are assessing the effect of TMS on decision making. Our data is not normally distributed because our assesment test results in mean count data (ultimatum game). Besides, ...
7
votes
2answers
190 views

Large number of correlations with MRI region of interest (ROI) variables: What adjustment should I perform?

In our group, we have measured grey matter volumes of about 20 regions of interest (ROI) and frequency of cannabis use. Now, we would like find out whether cannabis use is associated with grey matter ...
1
vote
0answers
162 views

Assumptions for robust linear regression, specifically ANCOVA type models

I have started looking into robust methods of data analysis. In particular, robust regression methods for ANCOVA type models using the "lmRob" function from the "robust" library in R. I'm having a ...
1
vote
0answers
267 views

How to perform ANCOVA on mixed experimental design where covariate is one level of a condition using R?

This is a second question related to previous post: Is this mixed ANOVA correctly specified in R? To reiterate, the setup is as follows: Groups: two groups of 8 subjects (16 total) Two conditions: ...
1
vote
0answers
741 views

How to perform non-parametric ANCOVA adjusting for differences in age and sex?

Context: This is the design: 11 patients were examined for a "Response Parameter" before and after a specific treatment. 12 healthy control patients were examined for the "Response Parameter" (did ...
1
vote
0answers
663 views

Help with pretest-posttest data analysis

My research topic is based on comparison of the effects of traditional and modern teaching techniques on students’ attitude and achievement in science. I followed a pretest-posttest experimental ...
0
votes
1answer
97 views

How do you model a scalar response using a scalar IV, a categorical IV and categorical covariate w/ interactions?

So I originally thought I would be able to solve this problem fairly easily with ANCOVA, but now that I've thought more deeply about it I realized it is not so simple. Essentially we have a scalar ...
5
votes
1answer
621 views

Should the difference between control and treatment be modelled explicitly or implicitly?

Given the following experimental setup: Multiple samples are taken from a subject and each sample is treated multiple ways (including a control treatment). What is mainly interesting is the ...
3
votes
1answer
245 views

Using ANCOVA with data matched on propensity scores

I've read papers comparing them, but never seen a study that used them together. Is this done? Why or why not? Suppose you use ANCOVA to analyze a reduced sample of matched pairs generated using ...
4
votes
1answer
275 views

What are the issues when performing an ANCOVA with a covariate that is influenced by experimental manipulation?

Context: I am doing a study on lying. The independent variable was manipulated such that participants were either told to lie or tell the truth. The dependent variable is related to the amount of ...
4
votes
3answers
4k views

When should one use multiple regression with dummy coding vs. ANCOVA?

I recently analyzed an experiment that manipulated 2 categorical variables and one continuous variable using ANCOVA. However, a reviewer suggested that multiple regression with the categorical ...
2
votes
1answer
108 views

Assessing effect of intervention where aptitude tests used in pretest and posttest have some overlapping and some distinct items

In my research there is a pretest, and a treatment followed by a posttest. The pre and the posttests are similar but not exactly the same. A few questions were changed between the pre and posttests ...
5
votes
2answers
467 views

Longitudinal relationship between chocolate consumption and happiness: repeated measures ANOVA?

I am using SPSS and having some trouble with a research question which is analogous to the hypothetical question: Is there a longitudinal Relationship between Happiness and Chocolate Consumption? ...
2
votes
2answers
467 views

ANCOVA with multiple instances of the between-subject covariate

If I find that my covariate (reaction time) alters over the length of my experiment (e.g. due to fatigue), can I somehow build that into my model? So what I am saying is that the effect of my ...
2
votes
1answer
309 views

Questions about number of groups and group size in planned comparisons in ANCOVA

I have three questions. I want to conduct a planned comparison in ANCOVA which compares three groups (n=2, n=7 and n=5) with two groups (n=11 and n=25). Is it ok to include the group with n=2 if I ...
1
vote
2answers
3k views

How to set up a two-way mixed ANOVA with a covariate in SPSS?

I wanted to carry out a two-way ANCOVA for my data. However, SPSS isn't liking that I have only one IV. I have One IV: Group (3 levels, i.e. 2 experimental groups and a control group), One DV: ...
7
votes
2answers
8k views

Analysis of covariance in R

How do you carry out analysis of covariance using R? How do you interpret the results? A practical example will be highly appreciated.
3
votes
1answer
485 views

Requirements to perform a two-way factorial ANCOVA

This is a question that landed on my desk, and I don't have the requisite experience to reply. A researcher has been asked to perform (by a journal reviewer) a two-way factorial ANCOVA on some ...

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