Analysis of Covariance.

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R - Test for homogeneity of regression slopes results in singular model

I am trying to check the assumptions of a two-way ANCOVA. So in my model I have two factors (F1, F2) one dummy coded two level covariate (C) one dependent variable (D) In order to check the ...
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12 views

Seeking covariates that can negate the effect size of treatment groups in an ANCOVA model

I was going to build an ANCOVA model with two treatment groups (climbed/unclimbed) and a set of 10 potential environmental covariates which may collectively remove the significance of the climbing ...
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17 views

How is the coefficient of determination in ANCOVA computed?

Given the following ANCOVA model: $Y_{ij}=\mu+\alpha_i+\beta X_{ij}+\epsilon_{ij}$, $e_{ij}\sim N(0,\sigma^2)$ i.i.d., $\alpha_1=0$, $i=1,..., m$, $j=1,..., n$ Given I have already computed ...
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9 views

SSCP in analysis of covariance

I want to use analysis of covariance in R. The model is $y_{ij} = \mu + t_i + r_j + b(x_{ij} - \bar{x}) + e_{ij} $ In R it is possible to get Ryy, Tyy, Eyy, SSE as well as b and adjusted treatment ...
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14 views

Normalization against Covariates

I have a list of parameters which correlate with 1-2 covariates that I want to control for. Following normalization, I wanted to do comparisons between groups, correlation analysis and probably use ...
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13 views

difference between mixed effect and a simple ANCOVA

So I have collected data for 50 years of my dependent variable (continuous) data (x), independent (continuous) variable (y) across 10 locations. My null hypothesis is that the regression slopes ...
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16 views

Covariates in Repeated Measures ANOVA

I have made a repeated measures ANOVA which I have two independent variables, three covariates which I will calculate the effects individually for my thesis. I basically conducted two tasks with the ...
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13 views

Estimated marginal means and means from descriptive statistics - interpretation in ANCOVA?

I ran an ANCOVA which proved that the covariate is significant. If i understood right, the estimated marginal means show what are the means of the DV when we account for the covariate. Then we could ...
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40 views

ANCOVA: Calculate $\omega^2$ for two factorial design

I am trying to calculate the $\omega^2$ effect size for a two factorial (with two levels each) effect size for an ANCOVA with one covariate. Following deliberations from Howell (2012) I got the ...
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21 views

ANCOVA in R with unequal sample sizes

I'm trying to assess the outcome of an RCT using R. Participants were randomly assigned to complete one of two online interventions for depression. I want to see if there is a difference in the ...
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16 views

testing for the difference in slopes if independent data is correlated

So I am looking at whether the effect of rainfall on crop yield for 8 different States is similar across these States i.e. I am testing whether the slopes between rainfall and yield is significantly ...
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29 views

R: What is the difference between ANCOVA and drop1?

In one of the slides of a statistics course that I followed the following about using drop1 or ANCOVA is stated. Using drop1 the p-values shown are p-values for deleting one variable at a time ...
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19 views

ANOVA, unbalanced designs, missing data, and multiple comparisons

I am having several problems with my dataset and how best to analyze it. I have measured a series of plant phenology characteristics (with a seperate model for each one - I do not want to combine them ...
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1answer
59 views

Which Statistical Technique (ANCOVA or Taguchi methods) for this problem?

I have one qualitative factor (Treatments) and two uncontrollable factors, temperature and moisture, with one response variable. I'm confused whether I should use ANCOVA with temperature and moisture ...
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36 views

Comparison of t-value and F-value

I have conducted a t-test via SPSS, followed by an ANCOVA (Analysis of Covariance). Now I am wondering if it is correct to report the t-value and then the F-value of the ANCOVA. But are those two ...
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13 views

Scale of control variable in ANCOVA

I am new to the field of ANCOVAs. I want to check if two groups differ in three quantitative variables. At the same time I would like to control for gender and diagnose (6 categories). Is it ...
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21 views

ANCOVA with complicated research design

I am having a few problems with my analysis and hope that somebody can help me out. About my design: I have 120 respondents, who I showed each 7 facebook posts out of a pool of 16 (randomly assigned). ...
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44 views

Help with a stats question (what analysis to use?)

A common design in research is a study which employs both a treatment and control group with each group being measured at pre-test and post-test. There are many ways to analyze this data to determine ...
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42 views

R: Post hoc tests in ANCOVA with factorial design

Does anyone know how to do post hoc tests in ANCOVA with a factorial design? I have two vectors consisting of 23 baseline values (covariate) and 23 values after treatment (independent variable) and I ...
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29 views

Odd result for intercept in ANCOVA

I try to perform an ANCOVA for data with two factors and one covariate. The first factor has 2 levels and the second has 4. ...
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25 views

How to report ANOVA F-values when they are zero?

I conducted an ANOVA and one of the F-values for a main effect is .000 on SPSS, and the other is .003 on SPSS. As I am rounding to two decimal places, how do I report these? Does F = 0.00, or is F ...
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20 views

Linear correlation in ANCOVA - necessary?

I've been told by my stats prof that running an ANCOVA relies on a series of assumptions, including that the the dependent variable and the covariate are linearly related. So, in SPSS the first thing ...
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63 views

ANCOVA with “Day number” of my study as covariate

I've got some problems with selecting right model/method in my analysis. Two groups of animals (differ by "Treatment") were measured from 1-st to 42-nd day, one common value for each group was ...
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97 views

ANCOVA: ordinal covariate on SPSS

I'm trying to run an ANCOVA on SPSS with 3 variables: Gender, score 1, score 2. I want to control for the possibility that gender has an effect the scores, in order to isolate just the relationship ...
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49 views

ANCOVA in R and setting contrasts

Using the following data, I need to determine if, independent of mass, the observed response is associated with sex, location, and/or age ...
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26 views

How to code and interpret a regression with 4-level categorical variable and continuous covariates

I have a problem in which 4 groups of people are compared in their performance on a certain test. One way anova shows that they are in fact different. But I also know that my groups are different in ...
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33 views

mixed between-within design when covariate-DV relationship is non-linear (quadratic)

My question is about what method to use for a mixed between-within design when the relationship between the covariate and the DV is non-linear (quadratic). My DV is reasoning task score. There are ...
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86 views

SPSS: How to decompose interactions in repeated measures ANCOVA?

I conducted a study with three categorical within-factors and one continuous personality variable: topic (1,2) x gain loss framing (1,2) x achievement security framing (1,2) x personality I know how ...
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51 views

Multiple covariates in an ANCOVA design

I have found a group difference (3 groups) for my dependent variable and now I am supposed to find out if possible covariates influence this finding. What is the best way to "correct" for 5 ...
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1answer
58 views

What type of test do I use?

I have created 39 question Likert scale survey and want to cross-reference results with 5 demographic questions. I plan on assigning values for each answer 1=strongly disagree, 2=disagree, 3=neutral, ...
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1answer
68 views

Perform an ANCOVA only if one-way ANOVA proves statistical significance?

I am new to statistics and I have been doing some reading on ANCOVA lately. There is something that is slightly confusing me: When motivating the use of the ANCOVA model, many of the resources that I ...
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142 views

paired t test vs linear regression vs ANCOVA

I have data on patients before and after an intervention, half have the treatment, half did not. A paired t-test seems a simple way to determine if there is a significant difference after treatment in ...
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41 views

What happens with covariates when doing contrasts?

I am doing an analysis of covariance (in SPSS) but can't find anywhere how does SPSS treat the covariates when producing the analysis for the special contrasts I specified. Does it take them at their ...
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159 views

Repeated Measures ANOVA or ANCOVA?

We recently conducted an repeated measures experiment consisting of two conditions. Each condition consistent of three tasks. The tasks are seen as random factors because they are not comparable among ...
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83 views

How to properly analyse longitudinal data from experiments?

I am wondering how one should properly analyse longitudinal data from simple experiments (not observational studies). Imagine an experiment where subjects are first measured at baseline, randomised to ...
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51 views

Data formatting for an ANCOVA in SPSS

was wondering if someone could help clarify an ANCOVA for me, or maybe post me in the right direction. I have a set of outcome measures that have been administered pre treatment and post treatment. I ...
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1answer
61 views

How to test whether the effects of predictors on one DV explains their effects on another in a mixed ANOVA in R

I have data from a psychology experiment in which instructional method was manipulated between subjects via two factors: pretraining (3 levels) and training (2 levels). I assessed participants' ...
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2answers
26 views

Compare regressions among more than 2 groups

I would like to know if there is a way to compare the relationship between a factor (length) and a response variable (lipid content) among multiple groups? I have given up on ANCOVA, as length is ...
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63 views

Can you put more than one factor in ANCOVA analysis?

I am running ANCOVA on SPSS using post-operation score (continuous) as Dependent variable, pre-operation score (continuous) as Covariate, and 2 groups (categorial 1&0) as Fixed factors. This is ...
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283 views

ANCOVA in observational studies: what are the assumptions?

Using ANCOVA when groups differ on the covariate is controversial, although Tabachnick and Fidell write that this is a plausible function of ANCOVA in quasi-experimental (or observational) studies. As ...
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102 views

interpreting ancova and extracting estimated means

Imagine the following data: ds <- data.frame(x=1:10,y=1:10,z=rep(c("A","B"),each=5)) the means for groups in z are: ...
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1answer
61 views

Differences in power in regression versus mean of a ratio

I've been futzing around with the following example, and feel that there must be a good explanation or reference that I am blindly missing. Supposer you have a linear relationship between two ...
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119 views

Can I use ANCOVA for mediation analysis?

I'm a student of psychology and currently drafting the experimental design for my thesis. I could really need some help with the following statistical question. In my experiment, participants will be ...
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30 views

Methodology for finding source of MANCOVA interaction

I am trying to find out the best method for determining the source of a significant MANCOVA interaction. I have 17 traits measured for four locations. Is there an alternate to univariate ANCOVAs ...
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69 views

What is my experimental design?

I am very new to mixed/multilevel models. I have an experiment where we measured 2 scale variables (varA and varB) in 2 different groups of subjects (control and treatment) at 4 different time points. ...
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2answers
120 views

What is the most appropriate test for a multi-year different group/yr experiment?

I've been working on a research project for close to five years now. For my thesis I have to show how "well" did my approach improve things. Setup: Every year we use a tool A to brainstorm and ...
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202 views

How to perform an Ancova with a covariate with nested factors and unbalanced?Is this analysis appropiated?

I'm trying to see differences between years (2008,2009) and between seasons (winter, spring, summer) for a quantitative variable (dependent) in this case is the number of eggs. This variable ...
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1answer
113 views

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 ...
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63 views

Several covariates in ANCOVA? Or difference scores?

I am running a quasi-experimental study where I compare changes in brand attitudes across time for four brands, all within subjects. Between Time 1 and time 2, participants see some information and i ...
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40 views

How to interpret results from an experiment where covariate is influenced by the experimental setup?

I am doing study investigating the effect of company's heritage on consumer attitude towards the company/brand. For this purpose I have created two texts: one revealing the history of the company, ...