Questions tagged [group-differences]

Group differences broadly refer to statistics which quantify the differences between two or more subpopulations.

Filter by
Sorted by
Tagged with
18
votes
5answers
2k views

What is effect size… and why is it even useful?

I have an introductory-graduate-level statistics background (assume I know mathematical statistics and probability at an undergraduate level (e.g., Wackerly et al., Ross' Probability), and have some ...
14
votes
2answers
47k views

Unequal sample sizes: When to call it quits

I'm peer reviewing an academic journal article and the authors wrote the following as justification for not reporting any inferential statistics (I deidentified the nature of the two groups): In ...
13
votes
3answers
7k views

Comparison of ranked lists

Suppose that two groups, comprising $n_1$ and $n_2$ each rank a set of 25 items from most to least important. What are the best ways to compare these rankings? Clearly, it is possible to do 25 Mann-...
11
votes
0answers
150 views

10 % false positives from nonlinear mixed effect models : Why? [closed]

I've run a simulation study in order to estimate type I error rate of the test of group effect in a nonlinear mixed effects model, using nlmer from lme4 package. The results show there is 8-10 % false-...
10
votes
2answers
3k views

Compare the statistical significance of the difference between two polynomial regressions in R

So first of all I did some research on this forum, and I know extremely similar questions have been asked but they usually haven't been answered properly or sometimes the answer are simply not ...
9
votes
3answers
16k views

How to test whether subgroup mean differs from overall group that includes the subgroup?

How can I test whether the mean (e.g., blood pressure) of a subgroup (e.g., those who died) differs from the whole group (e.g., everyone who had the disease including those that died)? Clearly, the ...
9
votes
2answers
188 views

Investigating differences between populations

Say we have a sample from two populations: A and B. Let's assume these populations are made of individuals and we choose to ...
8
votes
2answers
2k views

In a meta-analysis, how should one handle non-significant studies containing no raw data?

Let's say that I'm conducting a meta-analysis, looking at the performance of group A and group B with respect to a certain construct. Now, some of the studies that I'll come across will report that no ...
8
votes
3answers
1k views

How to compare ranked data?

I have some questions about how to analyze ranked data. The data looks like this: 4 groups of people with HIV and 16 other groups of people living in the same village were asked to rank 12 ...
7
votes
1answer
137 views

conditional difference between 2 uniform random variables. Surely Breiman can't be wrong?

I've already searched and don't find this particular case in XValidated. Statement: "Electricity is turned on uniform-randomly at a given time of day (in 24 hour window). Once it has been turned on, ...
7
votes
3answers
233 views

Are these populations non-random and different?

What would be the simplest, straightest-forwardest way to determine the following: Whether A,B,C is non-randomly distributed in each single group (IE: are there more A's in group 1 than random chance ...
7
votes
3answers
8k views

Test to prove two means do not have a difference

I have a population that I measure a value from once a day. I then made a change, and continued to measure that same value from the same population. What test could I use to prove that my change did ...
7
votes
2answers
962 views

Cook's distance in detecting outliers

According to my understanding, Cook's distance measures the influence of each observation by excluding points when fitting a model. So I assume it could be an reasonable approach for outlier detection?...
7
votes
1answer
4k views

Principal component analysis with group data

In my experiment I have used 30 different accessions of a species. One group is challenged drought and the other group is control. I have collected data on 6 different parameters. I want to know which ...
7
votes
1answer
110 views

Test whether time of maximum differs across two groups

Suppose I have 10 repeated measurements of a quantitative variable $X$ of the individuals within two groups (let's call them A and B). For each group, I have the estimated means of variable $X$ at the ...
6
votes
2answers
2k views

How to test group differences when neither parametric nor nonparametric assumptions are met?

Consider following example. We asked 50 men and 50 women one question and they answered on 5 point likert scale (1 is completely disagree and 5 completely agree). Suppose that 50 men responded 20 ...
6
votes
3answers
601 views

Intuition for the standard error of the difference of sample means

I read in Wilcox, 2003 p. 247 that the standard error of the difference between two sample means is (assuming the normality and homoskedasticity assumptions): $\sqrt{\frac{\sigma_1^2}{n_1} + \frac{\...
6
votes
1answer
271 views

Check for significant difference between numbers of sightings per cardinal direction

I have data in a form like this: quantity direction 10 n 5 e 6 ne 12 n 20 nw 5 s 8 n 1 sw 3 se 2 ne 6 nw 8 n 2 se 3 e 4 w 9 nw The information behind these ...
6
votes
2answers
593 views

Can the difference between the means of two groups lie outside the confidence interval for the difference?

I received this question from a colleague which left me with a desire to find out a bit more about it. If you have two groups A and B with means--for example, 7.9 and 5.8--is it possible that the ...
6
votes
1answer
1k views

ordinal regression or Spearman correlation?

I am conducting research investigating two different groups of women: one group of Short-Stature Women (SSW) and another with Non-Short Women (NSW). We have the hypothesis that SSW has an inaccurate ...
6
votes
4answers
19k views

Comparing two binary variables of unequal sizes

I measured a binary variable from two different populations, and now I'm trying to find out whether the different populations differ with regards to this variable. I could use a Chi-Square test, but ...
6
votes
1answer
1k views

Should I re-center variables when looking at moderator effect in men and women separately?

I want to see if an interaction variable in a multiple regression is significant for the whole sample, and then just for men and just for women. When I created the interaction variable for the whole ...
6
votes
1answer
4k views

Comparing distributions of unequal sample sizes

Consider the distribution shown in the below histogram: I have computed a Welch's t-test for a difference in means between these two groups, as well as a Kruskall-Wallis test to see whether these two ...
6
votes
1answer
623 views

Comparing unbalanced groups with ANOVA/Kruskal-Wallis when one group has only 1 observation

I would like to compare a continuous variable across 5 health status groups, but one of the groups has only 1 observation. Would an ANOVA/Kruskal-Wallis be valid? What can I do about the group with ...
5
votes
3answers
84 views

Mixed models. Random slopes only, mean and group centering?

Are random intercepts a theoretical/practical prerequisite to random slopes? Why? I have a three level (rep measures) mixed model where I wouldn't expect lvl 3 variation in initial status of outcome ...
5
votes
2answers
200 views

Claiming validity of a study's negative finding

For a disease that I am interested in, five or six published works of research have shown an association of changes in gene expression with presence of the disease. However, all these diagnostic ...
5
votes
1answer
9k views

Chi-squared test when two vectors have different lengths

I want to compare two groups (vectors) having different lengths to see if their distributions differ considerably. However, when I use chisq.test, I get an error ...
5
votes
3answers
860 views

How to compare two groups on a measure of social skills that includes 5 subscales where each subscale is number correct out of 12?

I have two groups of participants. Each group contains 30 children (15 girls and 15 boys). The first group has been sampled from a population of children being raised under some kind of constitutional ...
5
votes
2answers
7k views

How to determine the cut-point of continuous predictor in survival analysis, optimal or median cut-point?

everyone! I want to do overall Recurrence-free survial analysis for one continuous predictor with Kaplan-Meier method, but the determination of the cut-point for groups really confused me. Most ...
5
votes
1answer
5k views

Testing for differences between a sample and a subsample

For reasons I won't go into, I want to test for group differences between an entire sample A ($n=65$) and a subsample B ($n=45$). All of the variables I want use for mean comparisons are continuous. ...
5
votes
1answer
798 views

Significant difference between difference scores of 2 groups on two measures?

I am investigating a discrepancy between male and female self reports of sexual experiences. The original survey consists of a female version (asking about victimization) and a male version (asking ...
5
votes
2answers
279 views

Should false discovery be controlled at the data acquisition level, or should this be at the data interpretation level?

Should false discovery be controlled at the data acquisition level, or should this be at the data interpretation level? I have an experiment in which microarrays were used to quantify the expression ...
5
votes
1answer
51 views

Why does randomising the order of measurements remove time as a confounding variable?

Say we're interested in the difference in x between Group1 and Group2. We might measure 50 samples from Group1 then 50 samples from Group2. If the accuracy or precision of our measurements change over ...
5
votes
3answers
2k views

Statistical tests for uneven groups

We performed an experiment (usability testing), which had two independent samples, one group with 5 participants and the other with 24. Based on the analysis of the distribution of the data (Shapiro-...
5
votes
1answer
1k views

Power analysis for post hoc test of ANOVA with many groups

In both G*Power and the R pwr package, estimated sample size required per group decreases as the number of groups increase. This seems somewhat counter-intuitive. For a toy example, assume that I ...
5
votes
1answer
99 views

Reject $H_0: \mu_1 = \mu_2 = \mu_3$ but not $H_0: \mu_1 = \mu_2, H_0: \mu_3 = \mu_2,$ or $H_0: \mu_1 = \mu_3$?

Is it possible to reject $$H_0\!: \mu_1 = \mu_2 = \mu_3,$$ for example, with an F test in the context of an ANOVA table, but not reject any of the following: \begin{align} H_0\!: \mu_1 &= \mu_2 ...
5
votes
1answer
1k views

What is the equation for combining standardized mean differences across multiple studies?

I am trying to come up with a combined/pooled standardized mean difference for meta-analysis of a dozen or so studies that have the same binary predictive groups and the same continuous outcome ...
5
votes
0answers
568 views

Two-sample bootstrap?

I have two independent samples of observations. From each sample I produce a statistic. Let's denote these as $\theta_1$ and $\theta_2$. I'd like to test the hypothesis that $H_0: \Theta_1=\Theta_2$, ...
4
votes
1answer
208 views

Approaches to compare differences in means with differences in proportions?

I have summaries of hundreds of variables for two populations. Most of these variables are counts, but some are continuous. For example, if I had population sizes $N_0 = 20000$ people and $N_1 = ...
4
votes
2answers
3k views

Assessing group differences on multiple outcomes

I have one independent variable with two levels: Gender (Male and Female), and dependent variables will be composed of three test scores: Impulsivity, Assertiveness, and Social Desirability. My ...
4
votes
2answers
87 views

What is the variance of the standardized mean difference of two independent studies?

I have computed the standardized mean differences for a pair of independent samples. But when I Googled for the "variance of the standardized mean difference" I got rather confusing results, with ...
4
votes
2answers
689 views

Comparing different conditions on a binomial distribution

I have some data where I have tested a binomial random variable under 4 conditions. The null hypothesis is that they all have equal means, alternative hypothesis is that one or more means differ from ...
4
votes
1answer
984 views

Testing for equality of variance in permutations (microarray analysis with bioconductor)

I have measured whole-genome gene expression in two groups of animals, n=6 in each group. My goal is to detect differentially expressed genes - pretty standard analysis. The typical thing to do, and ...
4
votes
1answer
223 views

What is the relationship between differential analysis and hierarchial clustering?

I'm currently in an internship for R bioinformatics, where I'm writing software for single-cell RNA sequencing analysis. We're looking for differentially expressed genes between groups, but I don't ...
4
votes
1answer
481 views

Accounting for groups in GzLM Poisson regression

I am currently trying to perform a regression on data with one predictor and one covariate. The hypothesis I am trying to test is whether the incidence of pores (a hole through a cell) increases when ...
4
votes
1answer
240 views

How to evaluate differences of multiple Likert scale ratings of multiple items within groups?

I have data where different reviewers rated how important multiple topics are on a scale of 1 to 5 (Likert scale). Each of the topics is assigned to a group. I want to see if there any differences in ...
4
votes
1answer
537 views

Nesting and grouping formula confusions using lme

I've seen a fair few of these posts already, and I feel a bit silly for posting yet another question on the topic - but I just cant seem to get my head around it. I'm very new to R and esp new to lme, ...
4
votes
1answer
3k views

Comparing the result of a study which has unequal group sizes

I have conducted two user studies and in my studies I didn't have control over the group sizes. In each study users were put in groups and they were asked to perform some group activities. Here is the ...
3
votes
4answers
236 views

Plotting averages when there are missing values

I have count data for a number of subjects in different groups I would like to compare. The averages of the cumulative sums are shown in the figure below. As you can see the Red group has a spike at ...
3
votes
4answers
1k views

Grouping samples by clustering or PCA

If I have 5 binary variables with values for 100 observations to give me a 5x100 matrix. ...