Refers to the conditions under which a statistics procedure yields valid estimates and/or inference. Many statistical techniques require the assumption that the data are randomly sampled in some way. Theoretical results about estimators usually require assumptions about the data generating ...
1
vote
0answers
23 views
Transforming data with both negative and positive skew
I am attempting to run a MANCOVA of memory assessment data. My IV has three levels (no memory impairment, moderate memory impairment, and severe memory impairment). The covariate is education. The DV ...
1
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0answers
35 views
What are the assumptions for repeated-measures MANCOVA?
Are these assumptions correct and the list complete for repeated-measures MANCOVA?
sphericity
multivariate normality
homogeneity of variance
homogeneity of covariance matrices
homogeneity of ...
2
votes
1answer
80 views
Verifying the assumptions of the paired t-test
I have data on the number of cases that customers open (with an engineer) at a computer company each month, across the various technologies that are supported. The assumption is the documents are ...
2
votes
0answers
50 views
Violated assumptions for regression
I am not sure what to do with this regression residuals.
The Normal Q-Q plot is indicating that residuals are non-normal (not too extreme) and residual variance is not constant (not too bad). My ...
5
votes
1answer
81 views
Negative Binomial Regression - Assumptions?
I'm working with a large data set (confidential, so I can't share too much), and came to the conclusion a negative binomial regression would be necessary. I've never done a glm regression before, and ...
5
votes
3answers
82 views
Validity of normality assumption in the case of multiple independent data sets with small sample size
Due to limitations in experimental setup, I only have small data sets with n=3. Despite the low df the difference between treated and control is large enough to generate a significant p-value.
The ...
2
votes
1answer
61 views
Regression with non-normally distributed residuals
There are several posts on this site talking about the need of normality when interpreting the meaning of the p.value of a linear regression. But not much I think is said about how to deal with ...
1
vote
0answers
25 views
Importance of estimating $\sigma^2$ in linear Statistical model
Statistical model for Complete Randomized design
$y_{ij} = \mu + \tau_i + \epsilon_{ij}$
where, $i$ denotes treatment and $j$ denotes observation.
$i=1,2,...,k\quad and \quad j=1,2,..., n_i$
...
0
votes
1answer
42 views
Importance of the normality assumption in linear statistical models
Statistical model for Complete Randomized design
$y_{ij} = \mu + \tau_i + \epsilon_{ij}$
where, $i$ denotes treatment and $j$ denotes observation.
$i=1,2,...,k\quad and \quad j=1,2,..., n_i$
...
3
votes
1answer
56 views
Independence of samples
Recently I started doing verification on daily temperature forecasts by different providers. I calculated the absolute forecast errors for each of the providers, and I am planning on performing ANOVA ...
-1
votes
0answers
77 views
Is it fair to say that time-series violates IID? [closed]
First a little background.
It is said that, for the supervised learning framework. there should be a probability distribution over the input space, $P$ over $X$, (for example, as stated here ...
5
votes
2answers
62 views
Homogeneity testing of baseline characteristics in medical trials
I have been reading medical journals and they repeatedly show baseline characteristics of samples from a randomised controlled trial, which they have then tested to ensure no differences between the ...
2
votes
2answers
56 views
Assumptions and contraindications of conjoint analysis
I'm working with a team that is pressing hard to do a conjoint analysis on what is essentially a 2x2 factorial design. I get the feeling, though, that this is just their favourite hammer and not ...
0
votes
1answer
49 views
Model assumptions not met but model has predictive capabilities
This is a general question, has probably been asked before (I searched and did not see similar), may have a simple answer, may be ridiculous, and is just to satisfy my own curiosity.
Say we build a ...
1
vote
1answer
208 views
Zero conditional mean assumption
How can I check the zero conditional mean assumption for a multiple linear regression? I read that when the assumption is violated, your model is misspecified. And your estimators are biased. So we ...
5
votes
1answer
71 views
Constant variance assumption in regression model
My question is how do we check the constant variance assumption in a regression model?
2
votes
1answer
111 views
Can I justify performing a two-way anova where data is normally distributed but has heterogeneous variances?
I'm an undergraduate psychology student currently finishing up my dissertation, but a few days ago, while going through my SPSS output, I realized my Levene's test is significant, but my data is ...
0
votes
0answers
40 views
Assuming in a regression model that all off-diagonal elements of the beta-hat variance-covariance matrix are zero?
What are the implications of assuming in a regression model that all off-diagonal elements of the beta-hat variance-covariance matrix are zero?
Assuming all zero off-diagonals in the ...
8
votes
2answers
317 views
Where do the assumptions for linear regression come from?
I'v already known that there are several assumpations when using linear regression model. But I cannot understand why some of them exists. They are:
independent errors
normal distribution of errors
...
1
vote
0answers
54 views
Multiple Regression
I need help in this area for my thesis. I have 2 time lines in my research in which different variables are measured. The only variable that is the same for both timelines is on stress. It was ...
2
votes
1answer
31 views
Testing for variable independence with count data and assumption violations
I used a survey to collect information about injuries suffered while practicing and competing in gymnastics. I have count data on the injuries that looks something like this:
...
1
vote
0answers
35 views
Does the independence assumption interact with the generalizability of results in hierarchical designs?
A friend recently posed to me the following problem:
Is it possible to do a Chi-square on test results for a group of participants to determine if they made more of type A errors or type B errors?
I ...
0
votes
0answers
73 views
Model Trees: M5, Cubist - What about the assumptions of the linear models being used?
From what I understood, these models differ from CARTs for regression, mostly because they fit a linear model at the leaves of the tree instead of simply taking an average. They also "smooth" the tree ...
4
votes
3answers
192 views
Two significant covariates in coxph analysis: exploring whether the two covariates interact significantly
I am using R 2.15.2 GUI1.53 64-bit on MacOSX 10.5.8 to perform these analyses. I am a molecular biologist by training, not a statistician, so all this analysis represents my own bootstrapped learning ...
5
votes
1answer
425 views
Omitted variable bias in linear regression
I have a philosophical question regarding omitted variable bias.
We have the typical regression model (population model)
$$
Y= \beta_0 + \beta_1X_1 + ... + \beta_nX_n + \upsilon,
$$
where the ...
2
votes
2answers
246 views
Mann-Whitney for non-normal distributions with n>20?
I have 12 groups of datasets (mostly non-Normal, quite right-skewed), and I would like to see if there is a significant difference within each pair. (But the data itself is not a paired dataset, as in ...
3
votes
1answer
106 views
Level of analysis and assumption of independence in Pearson Chi-square test
I have survey data about injuries occurring to athletes over the span of their careers. Many of the athletes suffered multiple injuries (the most any athlete reported was five and this did not occur ...
2
votes
1answer
51 views
Properties of AR(p) model to be invertible
Suppose we are working with an $AR(p)$ model. We assume that $E( \epsilon^2 ) < \infty$.
We know that an $AR(P)$ model is invertible $\text{iff } \phi(z) \neq 0$ for $|z| \le 1$.
But why do we have ...
2
votes
1answer
339 views
Assumptions of correlation coefficient
What are the assumptions for the proper use and interpretation of the Pearson's correlation coefficient?
How is $\DeclareMathOperator{\cov}{cov}\cov(X,Y)$ affected by (mild) deviations of linearity? ...
1
vote
0answers
88 views
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 ...
3
votes
0answers
88 views
Exploratory data analysis
I have two data sets: one has a continuous response variable whereas the other discrete. I will use a linear model for the continuous data and a logit and probit model for the discrete data. What I ...
1
vote
0answers
28 views
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 ...
6
votes
1answer
384 views
Is ordinal or interval data required for the Wilcoxon signed rank test?
Having looked at multiple online sources, I can't seem to get a straight answer. Could someone please clarify for me if ordinal data is sufficient to use for the WSRT and if not, is the sign test an ...
1
vote
0answers
72 views
Assumptions for Hurst exponent calculation
Are there any general assumptions for the calculation of the Hurst exponent?
Does the signal need to be stationary, for example?
Does it depend on the method?
What about the length of the time ...
-3
votes
1answer
175 views
On what basis do we differentiate a standard normal distribution from normal distribution?
The normal distribution is a prerequisite for ANOVA. It is not clear what we are supposed to understand by "standard normal distribution". Does ANOVA need a standard normal distribution? The answer ...
2
votes
1answer
138 views
Should raw data or residuals be used to check homogeneity of variance?
Unexpectedly for me (!) I've recently learnt that:
"We have assumed that the error terms, $\epsilon_{ij}$, of the variates in each sample will be independent, that the variances of the error terms ...
4
votes
2answers
498 views
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 ...
5
votes
2answers
1k views
What to do when assumptions are not met in a chi squared test?
I am doing an analysis on a “a priori segmentation”. In order to examine significant differences between the two segments crosstabs and the chi-square test were used.
Type of variables: categorical
...
4
votes
1answer
237 views
When is it acceptable to use only two items (variables) per factor in factor analysis?
I have a set of 20 variables that I have put through factor analysis in SPSS. For purposes of the research, I need to develop 6 factors. SPSS has shown that 8 variables (out of 20) have been loading ...
0
votes
0answers
128 views
How do I test the assumptions of a independent two-way ANCOVA with SPSS?
I really need help in testing these assumptions regarding a two-way ANCOVA with SPSS:
Independent variables orthogonal to the covariates
Covariates must have a homogeneity of regression (or ...
3
votes
1answer
154 views
Assumption for an M/M/1 queue
When a queueing system is modeled as an M/M/1 queue, it is assumed that the arrival time of jobs has Poisson distribution and the service rate has exponential distribution. I am wondering what ...
1
vote
1answer
216 views
Independence of errors assumption in paired & independent samples t-test
first of all, this is a great site and I've already learned a lot from threads here wile searching for answers to my stats questions on the web. Yet here's another one:
I understand that the ...
1
vote
1answer
84 views
No-simultaneous-events assumption of the Poisson process
I am checking the description of Poisson process from Wikipedia.
The Poisson process is based on four assumptions, but I am not clear this:
No counted occurrences are simultaneous.
Can you ...
2
votes
0answers
118 views
How to implement a regression with non-independent observations in the DV?
I have a design where a number of object pairs were rated for association strength. These pairwise association ratings along 5 criteria of interest are expected to be used as IVs in a regression.
...
4
votes
0answers
49 views
When should one _avoid_ Bootstrapping? [duplicate]
Possible Duplicate:
What are examples where a “naive bootstrap” fails?
Perhaps this question will turn out to be a bit soft, but I think it may have a hard answer. Feel free to ...
1
vote
0answers
114 views
What is the nonparametric variant of multivariate ANOVA with more than one one factor (and with interactions between factors)?
I analyze real data where assuptions of classical multiway (more than one factor assumed) MANOVA (more than one response variable) are not met (normality, equal variances). Is there any nonparametric ...
0
votes
1answer
348 views
What are the assumptions of the Games-Howell multiple comparisons procedure?
What are the assumptions of the Games-Howell multiple comparisons procedure? Please back up any answers with references.
16
votes
4answers
7k views
Is there a minimum sample size required for the t-test to be valid?
I'm currently working on a quasi-experimental research paper. I only have a sample size of 15 due to low population within the chosen area and that only 15 fit my criteria. Is 15 the minimum sample ...
10
votes
3answers
994 views
What tests do I use to confirm that residuals are normally distributed?
I have some data which looks from plotting a graph of residuals vs time almost normal but I want to be sure. How can I test for normality of error residuals?
0
votes
2answers
413 views
Cox-regression model: question about acceptability, meaning, & reporting
Apologize for insufficient info in my previous version of this question. So, I am comparing patient survival in a retrospective study between two different groups, using SPSS. Now the difference came ...
