# Tagged Questions

Analysis of Covariance.

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### Assumptions of Quade Rank-Based ANCOVA

Spelled out in plain English, what are the assumptions of the Quade (1967) rank-based ANCOVA procedure? It's not written very clearly in the paper. For instance, in regular ANCOVA, I believe the ...
162 views

### Significant differences among fit lines - ANCOVA not enough?

I often am in the situation of having data sets consisting of an independent variable, a dependent variable, and a factor with multiple levels - for instance, calibration curves for an instrument ...
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### Ancova: What I can do when assumptions aren't met? [duplicate]

Possible Duplicate: ANCOVA and its disturbing assumptions I am testing whether prenatal smoking affects intelligence, while controlling for social economic status. I want to use an ANCOVA ...
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I have the dataset that contains one continuous measure of plant leafs (PC1 scores of individual leafs), that is dependent variable and three others that I need to use as independant, explanatory. One ...
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### mixed ancova or difference scores?

I am puzzled with how I should analyze the following data: The dependent variable is respiratory sinus arrhythmia, a measure of parasympathetic activity (RSA). I have a $5\times5$ full factorial ...
434 views

### Should I use ANCOVA or multiple regression with dummy variables?

I am writing a manuscript using an experimental design which predicts interactions between 1 continuous variable and multiple dichotomous variables, all predicting a continuous variable. As is ...
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### ANCOVA and its disturbing assumptions

It is possible to test for interaction in Analysis of Covariance (ANCOVA) between the independent variable and the co-variate, but isn't “the homogeneity of regression slopes” an assumption that ...
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### Generalized linear model as non-parametric ANCOVA vs. modern robust methods and non-parametric equivalant of ANCOVA

I am a medical student and I'm a rookie in the field of statistics but I was searching and digging for a long time to do a simple (for most of people involved in the field) analysis all by myself, and ...
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### Comparing model fits or regression coefficients for nonlinear models fitted to different data sets

I have kinetic data measured under several treatments (one experiment per treatment) which can be fitted by several nonlinear two-parameter models, how can I compare whether there there are ...
212 views

### How to specify ANCOVA interactions in SPSS?

I'm trying to run ANCOVA in SPSS, with a continuous IV as well as a continuous covariate. Variable Information: 1 DV: Continuous 2 IVs: 1 Continuous and 1 Categorical 1 Covariate: Continuous I ...
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### Baseline differences in RCT: Which variables (if any) should be included as covariates?

I recently completed a study whereby I randomly assigned participants to one of two treatment groups. I tested participants at baseline, immediately post-intervention, 1 months, and 4 months on a ...
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### ANCOVA: How does a covariate give insight when comparing the mean to the adjusted mean?

When comparing the direction and magnitude of the difference between the original means and the adjusted means what does the change say about the covariate? For example: IV = Grade, DV = Test Score, ...
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### Different ANCOVA results in R using different functions

before my actual ANCOVA I would like to test whether there is a significant interaction between the IV and the CV as this is one assumption for ANCOVA. I found 3 different ways in R to perform an ...
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### ANCOVA to check effect of covariate

I want to check whether a group of patients are significantly different from their control group. However, I also want to check if the p-value is still significant in case a covariate is taken into ...
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### Interpretation of intercept in ANCOVA

Consider the below ANCOVA example in which I am trying to predict a continuous varibale y by two variables x (continuous) and ...
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### How well can multiple regression really “control for” covariates?

We’re all familiar with observational studies that attempt to establish a causal link between a nonrandomized predictor X and an outcome by including every imaginable potential confounder in a ...
1k views

### Nonparametric equivalent of ANCOVA for continuous dependent variables

I have an independent categorical variable ($X$ with two categories, $x_{1}$ and $x_{2}$) and two continuous dependent variables ($y$ and $z$). Using a Mann Whitney test, I know that $y$ is ...
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### Choosing the best analysis

I need to do a data analysis in R. My data consists of 2 populations of animals that have 2 different types of genomes. The children are always born as twins. I also know the mothers' weight and the ...
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### Report coefficients of a factor in a GLM

I have a GLM and my explanatory variables are a factor of 9 levels (X1) and a numeric variable (X2). If I have to report the formula: $Y = B_0 + B_1X_1 + B_2X_2$ I have 1 coefficient for the ...
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### ANCOVA/ANOVA: Testing different models against each other

I have a dependent variable $Y$, an independent variable $X$, and a categorical independent variable T (which can take 2 levels). Now I have 4 models: MI: different slopes, different intercepts ...
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### Visual depiction of experimental treatments in (Poisson) ANCOVA?

Goal: I want to visually depict the difference between experimental treatments while controlling for a continuous covariate in Poisson regression. In my experiment, I am seeking to explain the number ...
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### Post hoc test of adjusted means (after ANCOVA)

When doing ANCOVAs, software usually gives both unadjusted means as well as adjusted means. However, the post hoc tests for most statistical programs are limited to unadjusted means. Yet, sometimes, ...
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### Is ANCOVA proper for my experiment?

I've got a serious problem when trying to analyse the data of my psychological experiment. Here are the details of the experiment: In each trial, subjects were presented with lateralized colored ...
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### Comparing means - 2 repeated measures variables, controlling for covariate with heterogeneous slope

I have a 2 level repeated measures DV of accuracy, and a covariate of response bias. As response bias increases, accuracy level 1 increases while level 2 decreases. I want to see if there is a ...
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### To exclude a main effect that is not of interest in ANCOVA, or not? [duplicate]

Possible Duplicate: Including the interaction but not the main effects in a model I have an experimental design with pretest and posttest. Two groups, experimental and control group. I am ...
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### How do I estimate a repeated measures ANCOVA in R and SPSS?

I'm currently running a perception experiment: DV: error (this in degrees- how much an observer was away from the real answer) IV: the time bin (5 levels) in which the unique stimulus appeared ...
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### Interactions with R? ANCOVA in R? I want to see if certain variables affect a dependent variable in R using repeated-measures

I have done an experiment in which the same people have been measured in two conditions, the same room with and without plants. I know they performed significantly better on a reverse digit span task ...
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### How to tell if a continuous variable is associated with (treatment) change in another variable over time (one group, time 1, time 2)?

Hypothetical: I have treated a sample for depression and have (continuous) depression scores at intake (Y1) and at the end of treatment (Y2). I am wondering if motivation (X) at the start of therapy ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### Ancova without intercept in R

Here is an exemple of fitting an ANCOVA model without intercept in R: ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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: ...
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### 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 ...
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### 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 ...