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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ANCOVA with count Data (GLM, Quasipoisson) and multiple Covariates (R)

at the moment I am working on my Bachelor Thesis and I got a problem in Data Analysis with ANCOVA in R. Unfortunally the experimental Design leads to a small number of samples. I got 12 Days and 3 ...
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ANCOVA or Repeated measures ANOVA or GLMM or GEE

I've got the following problem: background A unwanted species keeps entering the farms. Some farms might be entered more often than others. There are 80 farms in total, subdivided into two groups: ...
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Two-Way ANCOVA: It is necessary to include a non-significant interaction term in the model?

The objective is to test whether the adjusted group means are equal. The question is whether ANCOVA requires the interaction term, for this objective. In all the guides I am reading, after testing for ...
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Seeking Explanation of how this ANCOVA result in R shows the group means are not equal

I am looking for an explanation as to how the ANCOVA results, below, show that the means of the groups defined by IndVar1 and IndVar2 are not equal, after accounting for the covariate, Covar. I think ...
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Example 2.2.2 fromAn Introduction to Generalized Linear Models. Show $Y_{j k}, \bar{Y}_{j}, \hat{\beta}_{j} $ are all independent

This question is related to what I have asked in my previous post: How to derive the covariance matrix between $\bar{y}$ and $\hat{\beta_c}$ where $\hat{\beta_c}$ is the OLS estimator of a linear ...
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How to adress the -0.71 correlation artefact in the pre-post analysis of change from baseline in non-randomized trials?

Let me make some introduction first. It's well known, that in randomized trials, in the pre-post trials, by the formal pharmaceutical regulatory guidelines, both raw post and change scores should be ...
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ANCOVA post vs. pre analysis on a randomized trial - unequal slopes. How to interpret and report the main effect?

Let's assume I have two randomized trials comparing a new treatment to a reference one, illustrated below with 2 different approaches. I will use R notation. ANCOVA: post ~ pre * treatment (we want ...
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1 factor, 1 covariate, single control group. Is ANCOVA possible?

Let's say in our experiment we have three drugs (factor), each in 3 doses (covariate), with a separate group of subjects assigned to every drug-dose combination, and a single control group (no drug). ...
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Can regression to the mean be corrected by linear mixed effects?

I am wondering if regression to the mean can be corrected by using linear mixed effects models in the following case. ...
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Significant covariate interaction in two-way ANCOVA

I am doing a randomised experimental study in which I have two between group factors (let's call them valence of manipulation and type of manipulation). I am measuring my outcome variable before the ...
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How to interpret this prepost ANCOVA test result?

I have a basic pre-post trial design. Two randomized Groups and two tests for each participant in each group one prior to the intervention (here V1) and one post (V2). I am completely new to this and ...
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Appropriate effect size to report for permutation tests (Anova specifically)

Original question, with explanation of outcome and explanatory variables included. Suppose I have a neuroimaging dataset, e.g. diffusion tensor-derived measure for multiple white matter bundles (or ...
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What is the best analytic approach for two arm randomized pre-post trial?

Study design is a classic pre-post design: two trt arms (treatment, control). Subjects are randomly assigned into two arms. outcome is continuous and measured at baseline and a follow-up time point (...
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How can I perform a regression analysis for two groups?

I have a problem where I have three groups (responders versus non-responders at baseline, responders versus non-responders at week8, and responders at baseline compared to responders at week8), in ...
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Longitudinal changes: linear mixed effect model Vs linear model using baseline measure as covariate

I am trying to study longitudinal changes in patients for measure 'a'. I have 2 measures of 'a' at baseline ('a_baseline') and ...
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Controlling the influence of variation in a factor

I sampled communities in different habitats along a continental margin. Depth is a strong structuring factor in the ocean. To mitigate or control the influence of the depth factor and in a comparison ...
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How can this model be fit using R's lm function?

I have data that consists of a grouping variable $\tt{grp}$, a predictor $\tt{x}$, and a response $\tt{y}$. There are three groups. I want to fit to this data the model $y = \alpha_i + (\beta + \...
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Kruskal-Wallis test allowing for one covariate?

As a non-statistician (I am human geographer, interested in health geography), I am trying to solve a problem (using R). Let's say I have a simplified database called df which contain these variables: ...
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Difference between Repeated measures ANOVA, ANCOVA and Linear mixed effects model

What is the best way to analyze these data: Subjects are divided in two "Group" (Treatment A and B). "Weight" is recorded before and 3 months after treatment. Outcome variable: ...
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ANCOVA with Pre and Inter as covariate

I have 3 groups (CTL, Exposure A and Exposure B), an outcome measured at 3 moments (Pre, Inter, Post), and I am interested in comparing the effectiveness of each exposure at short (Inter) and long (...
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Appropriate statistical test to analyze differences in slopes of time series data in R

I have created the following plot based on air quality data over three years of observation, and would like to know if these slopes are different across the two time periods (March-June 2018-2019 ...
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Comparison of values by group with covariance

I have 3 groups of different sample sizes with covariance I need to include. I wish to evaluate the value dependence of the groups compared to group 1 (group 2, group 3 values compared to group 1). I ...
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Nested data and data analysis options

I want to compare patient experience (continuous DV; N = 60) for three different treatment modalities (face to face, phone, chat; between-subjects variable, one measurement). The sessions were run by ...
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Interpreting ANCOVA results

independent variable gender. Gender is a binary column consisting of 0 and 1.From the linear model summary below my observations are: Gender is significant for height dependent variable. Which means ...
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Factor analysis when I have mixture of continuous and dichotomous variables

I am trying to determine if there is a significant difference between two groups for a single continuous variable while accounting for baseline differences in dichotomous variables in each group. To ...
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How do I perform and interpret an Ancova/Poisson Regression, with multiple covariates and possible interactions?

I am researching how recovery time of trees after tropical wildfires differs between South America, Africa and Australia, using R. For this, I am using a Poisson regression. First I checked if there ...
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Two-group “ANCOVA” with Theil-Sen?

("ANCOVA" might typically refer to using at least three groups. In what I do, I am always comparing two groups, and that is what I mean throughout this question.) I know how to do ANCOVA via ...
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If interested in common a trend in multiple groups, can I pool all the data and run a correlation rather than an ANCOVA?

Dear CrossValidated Community, I have three groups, each of which has some biological data I'll call X, and I am interested in whether X is correlated with their treatment response, Y. Thus far, I ...
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How to interpret the interaction term in an ANCOVA model

Just looking for some help on my ANCOVA analysis. My study is basically looking at the difference in variables at 12 weeks in an exercise group and a control group. In figure 1 included as I ...
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Anconva or Anova for multifactor analysis

I have non-normal plankton community data, where rows are sites and columns are species (dependent) and a few factors. I want to check the influence of season on community (dis)similarities but am ...
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Discrepancies in robust ancova calculated in R with the WRS2 package vs by hand

I've conducted a study where participants in 2 groups (Control and Experimental) completed three scales (NJ, A, and D) at two time points (T1 and T2). I'm running ANCOVAs with the T2 responses as my ...
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Why is the Tukey method not appropriate for covariance analysis when estimating treatment effects?

In the book 'Applied Linear Statistical Models' chapter 22 it states that the Tukey method is not appropriate for covariance analysis. The context is in estimation of treatment effects. Instead, when ...
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Interpreting repeated-measures oneway ANCOVA

I have a question on how to run a repeated-measures oneway ANCOVA. I have only one within-subjects factor (time) and I am interested in how a construct changes from T1 to T2. I would also like to add ...
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Comparing two correlations/regressions with a repeated variable?

apologies this is possibly a repeat question but I have done a fair amount of googling and the answer is not obvious to me. I have two variables each measured on different days. Say a_day1, a_day2, ...
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Levels of a random effect coincide with values of covariates

My data are the wing area of insects, catched during the 12 years. Each year corresponds to one value of a mean ambient temperature, one value of rainfull and sample of the individuals. My task is the ...
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Homoscedascity tests for Two-Way ANOVA (a factor and a covariate or two factors)

I have the following formula: glm1 <- glm(y ~ g1 + g2, data = fish, family = binomial) y is a binomial variable: num 1 1 0 0 1 0 0 0 0 1 ... #There is (1) or ...
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Correct method of creating a repeated measures ANCOVA in r

I am attempting to build a 3x2 repeated measures ANCOVA in R where the same participants were measured 6 times using 2 different intervention combinations. 6 demographic covariate questions were ...
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ANCOVA with multiple covariates: How to test for homogenity of regression slopes in SPSS

I want to use ANCOVA to test whether means on a dependent variable (DV) differ between 3 groups (the independent variable, IV) while adding 3 variables as covariates (CV). One assumption of ANCOVA is ...
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Is it customary to report a beta coefficient for ANCOVA?

I'm reviewing a paper for a journal, and the authors have reported results of ANCOVA models. They report the F statistic, partial eta squared, beta coefficient, standard error, p value, and R squared. ...
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ANCOVA, Covariates, & Variable Selection

I just want to preface this question by saying: I've searched extensively for answer elsewhere without luck. If, however, you think this question has been answered elsewhere please point me in the ...
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How to test the disturbance effect of a measuring instrument?

I am trying to think of an experimental design that allows me to test to what extent a measuring tool (automatic counting unit) alters (by restricting the entrance area) the measured quantity (number ...
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ANCOVA $\eta^2$ definition

I am struggling to find references to understand what is the exact definition of $\eta^{2}$ is two way anova. Let's say I am estimating the following ANCOVA model: $$ y_{ijk} = \mu + \alpha_i + \...
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What type of ANOVA test should I use?

I have 2 independent categorical variables with 2 groups each, say ind_1 -ind_1a -ind_1b ind2 -ind_2a -ind_2b The outcome variable is quantitative and is measured at ...
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Testing percentages when I may or may not have the original values that give the percentages

I've found tons written about testing proportions between two groups (Fisher's exact test, $\chi^2$-test, logistic regression, etc) but very little on what happens in a t-test-like setting where the ...
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ANCOVA when response variable is calculated from 2 covariates

I have an experiment in which I apply a treatment (g), and measure variables X and Y; the response variable (density; Z) is simply X/Y. If I wanted to know if X or Y, when shifted under the treatment, ...
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Test if intercepts in ancova model are significantly different in R

I ran a model explaining the weight of some plant as a function of time and trying to incorporate the treatment effect. weight ~time + treatment The model looks ...
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MANOVA or repeated measures anova or ANCOVA?

for my thesis I want to compare 3 training groups (2 different types of training and 1 control group) to see the influence on mindfulness-skills and exhaustion level (so 1 IV and 2 DV). All the ...
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Interrupted Time Series Analyses in R with Type-2 Sum Squares ANCOVA Lagged Dependent Variable model

I have a short times series (less than 100 data points). I wonder how to compare effects of intervention I on values of time series. I prepared model on dataset Nile available in R. Let's assume that ...
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AN(C)OVA - Unequal variances even with log transformation

I would like to run an ANCOVA testing the mean difference in # of drinks (DV) consumed by students in 2017 versus students in 2019 (2 groups). Groups are independent and sample sizes are not equal (7,...
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ANOVA: Can a very small Residual Sum of Squares be in part due to a dependent variable of small magnitude?

Substantive Question: Could a very small Residual Sums of Squares be due in large part to a variable of small magnitude? If so, is multiplying the variable by some number (e.g. 10) an appropriate ...

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