Statistical Analysis Stack Exchange
Statistical Analysis Stack Exchange

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.

ANCOVA is used in ANOVA-like settings to control for any independent variable, which although not of interest, may affect the relationship between the dependent variable and other variables in the model. For example, to test the effect of asbestos on lung cancer, smoking would be included as a covariate.

ANCOVA is not substantively different from the General Linear Model: the term 'ANCOVA' may be used when a predictor of interest is categorical and it is wished to control for the effects of one or more continuous predictors; the sums of squares for the former are examined after partialing out those for the latter.

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Best practice when analysing pre-post treatment-control designs

Imagine the following common design: 100 participants are randomly allocated to either a treatment or a control group the dependent variable is numeric and measured pre- and post- treatment Three obvious options for analysing such data are: Test…
Jeromy Anglim
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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 multiple regression model. By thus “controlling for” all…
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Should covariates that are not statistically significant be 'kept in' when creating a model?

I have several covariates in my calculation for a model, and not all of them are statistically significant. Should I remove those that are not? This question discusses the phenomenon, but does not answer my question: How to interpret…
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Good resource to understand ANOVA and ANCOVA?

I am running experiments for a paper and I am looking for an interesting book / website to understand properly how ANOVA and ANCOVA work. I have a good math background so I don't necessarily need a vulgarized explanation. I'd also like to know how…
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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 variable coded as dummy variables is a more appropriate…
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Good data example needed with covariate affected by treatments

I have looked at a lot of R datasets, postings in DASL, and elsewhere, and am not finding very many good examples of interesting datasets illustrating analysis of covariance for experimental data. There are numerous "toy" datasets with contrived…
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How to perform an ANCOVA in R

I want to perform an ANCOVA analysis of data concerning density of plant epiphytes. At first, I would like to know if there is any difference in plant density between two slopes, one N and one S, but I have other data such as altitude, canopy…
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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 people are randomly assigned - categorical) This is a…
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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 a Gaussian error distribution with an identity…
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What is the difference between “factors” and “covariate” in terms of ANCOVA?

I am a bit confused on the term "covariate". It seems like the term can mean two different things. In ANCOVA, the term is used for the third variable that is not directly related to the experiment. For example, the age or IQ on the performance study…
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Post hoc tests in ANCOVA

Question: What is a good method for conducting post hoc tests of differences between group means after adjusting for the effect of a covariate? Prototypical example: Four groups, 30 participants per group (e.g., four different clinical psychology…
Jeromy Anglim
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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 difference between the control and each treatment. I can…
Rónán Daly
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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 group have a non-zero slope and non-zero intercept and…
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2x3 ANOVA interaction no longer significant after including a covariate?

My experiment had a 2x3 design with one covariate. If I analyse the results using an ANOVA, I get a strongly significant interaction between the two main factors (p<.001). If I add the covariate into the analysis and do an ANCOVA, the interaction…
Tomas Engelthaler
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What is the intuition behind (M)ANCOVA and when/why should one use it?

As per my understanding here's the what/why/when of the following hypotheses tests in a crude sense: t-test: Used when comparing means between two samples ANOVA (one way): Used when you have one dependent variable and one independent (i.e.,…
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