Questions tagged [ancova]

Analysis of Covariance. This is really a special case of multiple linear regression, used in ANOVA-like settings with some continuous covariates in addition to the categorical ones.

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Interpreting interaction effect with orthogonal polynomial contrast codes

I'm running a logistic regression with two categorical variables (sample_number: 1, 2, 3, 4; and sample_length: 60, 70, 80, 90, 100) and a continuous predictor (dot_size). My model in R looks like ...
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Control covariates in paired t-tests (pretest-posttest design)

Suppose we have a naïve single-arm pretest-posttest design without any control group. Every subject has a pretest score and a <...
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Is it ok to drop a non-linear treatment level in ANCOVA?

I am running an ancova to measure differences in an allometric relationship between tree height (dependent variable) and diameter (covariate) among one nutrient treatment with 4 levels. One of the ...
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Appropriate model choice for analyzing many repeated measurements in the context of a parallel arm randomized controlled trial

Background I am conducting a parallel-arm randomized controlled trial comparing Treatment A against a Control. Each week for 10 weeks I measure anxiety in each group each week (giving a total of 10 ...
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ANCOVA (or other model) with non-parametric data distributions in R?

I am working with a dataset collected from human subjects that has 46 continuous dependent variables, one categorical independent variable with four groups (n = 20-30 per group), and three covariates (...
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How to determine if one variable affects the relationship between two separate variables

I have a dataset where two continuous variables are almost perfectly correlated. There is a third categorical variable and I need to perform a statistical analysis to determine if the categorical ...
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Why type III sum of squares is not recommended to use when there is significant interaction?

I did notice that in the following example, I obtained a insignificant main effect for treatment when including interaction, although the treatment is clearly effective. Any intuitive explanation? ...
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Different ways to include pre-test performance as a covariate in a linear mixed-effect regression. Which is correct?

I have a pre-post experimental design, where I have measured participants' performance in three courses (tasks; A, B, C) at both pre and post-test. The study is a learning experiment. 60 elite alpine ...
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Should I use an ANCOVA test?

For this data set: I want to see if the DNA levels differ if there is food or no food and if this changes due to the number of days. Does it make sense to use ANCOVA as the Age could be considered a ...
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Effect of getting closer to a normally distributed covariate on result of ANCOVA

I have a question about ANCOVA. We did an experiment with 2 groups and then calculated an ANCOVA to see if the DV differed between the groups. We included one binary and one continous covariate (PO). ...
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Can this be classified as an ANCOVA?

ModA <- lm(Temperature ~ Section*sin(2*pi*t)+Seccion*cos(2*pi*t),data=MyData) ModB <- lm(Temperature ~ Section+sin(2*pi*t)+cos(2*pi*t),data=MyData) I have to ...
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Covariate analysis comparing distributions / means of circular data

Are there publicly available software packages to run an analysis comparing the means or distributions of two samples of circular data with the presence of a covariate? If not, are there ways to ...
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ANCOVA when continuous predictor is a main factor of interest: extension of a related post

From the literature, I understand that we often assume an ANCOVA is only useful for "controlling" for covariates. However, from this related Stack Exchange post, an ANCOVA should also be ...
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Multiple correlated outcomes in a randomized experiment with two groups and two time points: How to analyze?

In a randomized trial, subjects are randomized into two groups, a control group and an intervention group. There are multiple outcomes (15 in total), namely miRNA (microRNA) which are measured before (...
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Hurdle model: How to compare slopes?

I'm analyzing the change in seagrass cover (count data of shoots) between two years of different species. Therefore, I made a model similar to this: ...
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Baseline adjustment when both pre- and post values are subject to left-censoring

Assume $X$ is a binary treatment variable, $Y$ is a continuous variable measured pre- and post-treatment $(Y_{pre}$, $Y_{post})$, and $Z$ represents the remaining covariates. Both $Y_{pre}$ and $Y_{...
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Leave out non-significant covariate [duplicate]

In my study, I make use of two covariates. After running a two-way ANCOVA in SPSS, it turns out that only one of the two covariates is significant. Should I remove the one that is not significant, and ...
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GPower: Difference in Sample Size for ANCOVA vs. Repeated Measures ANOVA in clinical trials

I want to conduct a randomized pre-post intervention study with two measurement points (1. Baseline 2. Post-intervention) and two groups (Treatment vs. Control) and am currently trying to work out the ...
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Query regarding Models in MANCOVA?

This is the query in the area of Mancova in SPSS. There are 8 dichotomous IVs and 7 DVs. IVs are shape components of the bottle like deep curve, slim, parallel etc. and DVs are like sophisticated, ...
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Interactions and/or covariates for a regression in r

I have to find the regression between to continuous variables, and this I have done with an lm() in r. Now I want to test if a 3rd variable is a covariate, the variable is size and it is also ...
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How to statistically account for the different length of time patients were on the different drugs, as to compare the two groups in R?

Conducting a statistical analysis of two groups of subjects being on different drugs. One group has stayed on drug A, while the other group switched to drug B from drug B at around the middle of the ...
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What statistical method would I apply?

Basic information for my project is below; RCT Group(3); A type Exercise, B type Exercise, No(Control) Time(3 times measurements); baseline, 4wk, 8wk Outcome; mental health (continuous variable) -...
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What statistical model should be used to analyze data from a randomized pretest-posttest controlled trial when subgroups exist?

I'm planning a randomized pretest-posttest controlled trial of a psychological therapy intervention group compared to an active control group on a dependent variable of interest. My population will ...
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Analysing a controlled pre-post intervention study

I am conducting an analysis involving a non-randomised group of 30 individuals who received an intervention with the purpose of improving medicines regimes and a matched 1:1 control group as part of a ...
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How to simulate pre-test post-test trial data in R or SAS

I am in dire need of your help. I have to simulate a clinical study where the primary endpoint is change from baseline. The analysis method used in an ANCOVA. I am interest in mean difference between ...
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How to set Ancova model for following data and hypotheses with attached dataset

How to set Ancova model for following data and hypotheses with attached dataset Data set :
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Bootstrap ANCOVA/regression: what to do with residuals from different groups?

I find myself wanting to solve a standard ANCOVA-style problem that has a factor with two levels and then the covariate(s). Rather than standard methods of doing inference on the group parameter, I ...
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ANOVA is to ANCOVA as Kruskal–Wallis is to...?

ANCOVA is an ANOVA with an added covariate, and I find it quite elegant how both can be seen in a general regression framework. If, however, I want to do a Kruskal–Wallis-type of test that accounts ...
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Covariate significantly different among groups

I ran a 3-way ANCOVA with the covariate "depression level" (independent variable is anxiety level divided categorically as low/high). However, halfway throughout the analyses I noticed that ...
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Adjusting for continuous covariate in two-way ANOVA

I hope this question is not too basic, but as I am not a statistician I would really appreciate some input. Basically, I have the following problem: I have a continuous dependent variable (FA). All of ...
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ANCOVA assumption independence violated

I need some additional advice on my study. I plan to look at differences in constituents of several vegetable varieties. Now, this could be solved with a simple Anova, but my colleagues want to ...
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Can effect of intervention on outcome variable be inferred from ANCOVA or unstandardized path coefficient?

I'm fairly new to statistics, so the answer to this is hopefully obvious for some of you. I hope this is the right place to ask.   I'm currently having some difficulties interpreting the results of a ...
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Using PCA to combine variables in a randomized trial with a baseline and a follow-up measurement

Assume that $2n$ participants have several variables measured at two times points $t_1$ (baseline) and $t_2$ (follow-up). For illustrative purposes, I will use the example of just two variables: the ...
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checking reliability of covariate before running ancova

Many books (e.g., Stevens, 2007, chapter 7) emphasize that we must check the reliability of a covariate before using it in an ANCOVA procedure. Say I have a continuous covariate called ...
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What are appropriate covariates in ANCOVA? can you control for within-group variation?

I am comparing traits between pinned museum specimens and beetles collected in the field. So my independent variable is sample (historical/current). I'm using ANCOVA with sex and body mass as ...
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Do I need a two-way MANOVA?

I am testing for differences between 3 groups. I have 2 dependent variables which are continuous data. However, I am confused about my independent variables and if a two-way MANOVA test would be ...
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Questions concerning visualizing model results with the R-package visreg

Following some discussions with colleagues, I'm seeking for clarification on how to visualize an ANCOVA with visreg, and what the visualization actually shows. I would like to run the following ANCOVA ...
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Which type of sums of squares does lm-function in R use?

I ran a two-way ANCOVA in R: ancova = lm(DV ~ IV1*IV2 + CV1 + CV2 + CV3, data = Data) summary.aov(ancova) Anybody know if this uses type III sums of squares? I ...
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What type of analysis to do for Dissertation- EMOTION RECOGNITION

In my study, participants take part in the State Trait Anxiety Inventory (STAI) where they are ranked into high or low trait anxiety groups. Participants are then shown multiple videos of 7 different ...
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Why not accounting for a factor unrelated to treatment makes p-values too conservative?

I would like to know if a treatment works, but different subjects are measured across different sites. The site is unrelated to treatment, but it is related to the response (see the figure). If I put ...
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How to test the null hypothesis that two linear regression lines have the same Y value for a particular X value?

I am using ancova to test if two linear regression models are the same and it works great. I am following this tutorial: http://www.biostathandbook.com/ancova.html and I apply it in R with this ...
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Non-parametric equivalent to ANCOVA

I would like to do analysis with co-variate. However, some of my data is skewed and has outliers, so I would like to use non-parametric approach. The issue is that, I could not find any non-parametric ...
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How to look covariate for treatment between patients and controls?

I have this study where we have 2 patient groups, both on a the same medication (for ethical reasons, patients had to remain on their medication), and 1 control group. There are significant ...
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Mixed ANCOVA: What to interprete?

I've used several statistics packages to run a mixed ANCOVA with one within-subjects factor, one between-subjects factor and one covariate and I am confused as the output produced varies. When I run ...
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GLMM appropriate for constant covariates/ baseline score? Or Ancova?

I was wondering whether my data is appropriate to fit a glmm (with r package lme4) So, I measured a questionnaire two times (q1 and q2). Also, I have variables ...
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Analysis of treatment effect with partially non-randomized groups

I have conducted an experiment in which participants were randomly assigned to six groups: Control group Control group with additional information about problem Pre-set treatment group Pre-set ...
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How to control for a categorical confounding variable in statistical analysis without using parametric tests?

I have around 150 variables of psychometrical data for 97 subjects (Scores and percentages of tasks, Reaction Times, and Factors, such as social-economical status of participants). The goal of our ...
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ANCOVA for finding relationship between 1 categorical, 1 continuous IV and 1 categorical DV?

I have collected crime data from a few different sources. These sources label crimes differently, and I would like to see if I can safely combine any categories from the different datasets. For ...
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ANCOVA controlling baseline in RCT - power analysis and sample size questions

I am performing a power analysis/sample size estimation for an RCT. We will be controlling for baseline symptoms and using post-treatment or change scores as our outcome variable. This design is ...
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Would very small stratum be a problem for ANCOVA model?

I know that for logistic regression, if you get an empty cell, the model may not run at all. How about continuous outcome? If one of the categorical predictors has very small stratum, would there be a ...

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