An inquiry into the quality of a statistical test by calculating the power - the probability of rejecting the null hypothesis given that it is false - under certain circumstances. Power analysis is often used when planning a study to determine the sample size required to achieve a nominal level of ...

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1answer
18 views

Can Inferential Statistics be carried out in a Retrospective Case Study?

Just wanted to check with regard to a retrospective case study, if various data is collected for a set of patients on different variables can this be interpreted with inferential stats tests? Even if ...
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0answers
23 views

Retrospective Case Study

I am carrying out some research and am a novice to statistics, having done a 2 week module primarily focusing on parametric data and experimental design. However, for my research I am doing a ...
2
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0answers
18 views

Noncentrality Parameter - what is it, what does it do, what would be a suggested value?

I have been trying to brush up on my stats knowledge, especially in relation to Sample size determination and Statistical Power Analysis. But it seems that the more I read the more I need to read. ...
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0answers
42 views

How to modify post-hoc power analysis simulation to perform a-priori power analysis?

I am interested in doing a sample size calculation for exacerbation rates. I am drawing my data from a negative binomial distribution and my test statistic is based on a negative binomial glm. ...
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2answers
24 views

Power calculation for a test of trend over time

This paper (open access) Babies born at the threshold of viability: changes in survival and workload over 20 years (table 2) presents data for survival of extremely premature babies over the last 20 ...
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0answers
32 views

How to compare the price of two sets of business transactions?

A business transaction can be defined as a function of 6 independent categorical variables (representing the customer and the product). Sales representatives are allowed to charge whatever price their ...
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1answer
44 views

How can I find a level $\alpha$ most powerful test?

Let $X_1,X_2,\dots ,X_n$ be a random sample from a distribution with pdf $f(x,\theta)$. Find a level $\alpha$ most powerful test of $H:\theta=\theta_0$ against $K:\theta = \theta_1$ when ...
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0answers
20 views

Power analysis in a linear model - pooled vs individual samples

I'm interested in how many samples are required to see a relationship between allele frequencies (proportions of alleles) and environmental gradients, such as temperatures and water quality. ...
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0answers
52 views

How to compute sample size to detect a small change?

Suppose I wish to calculate the sample size needed to detect a one percent change in a proportion that is normally at 35%. I don't mean 35% to 36%, I mean 35% to 35.35%. Here is the calculation in R ...
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0answers
28 views

Sample size / power calculation for logrank survival test

I have really little idea of survival analysis. I am aware that logrank is a special case of Cox' proportional hazards model, and that tons of R packages and scripts address the problem of power / ...
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2answers
80 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 ...
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2answers
54 views

SD larger than mean, non-negative scale

I was given an article reporting a study very similar to one my lab wishes to run. But, I noticed that for the variable of interest, Duration, the SDs are larger than the mean...since this is ...
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0answers
35 views

Sample size calculation for logistic regression using simulations

I want to find the sample size for logistic regression where I have a covariate with 15 levels and the covariates interacts with time, which means that the effect of the covariates is different for ...
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0answers
24 views

Joint power calculation for two correlated test statistics

I have a situation where I want to perform two tests, where the null hypothesis is rejected if test 1 is significant and test 2 is not significant. If the two tests are independent then I think the ...
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0answers
57 views

power/sample size for a complex set of equations

I run an experiment in which several teams perform several tasks each (not all teams perform all tasks, but in general, I can make sure that enough teams perform any given task, and that each team ...
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0answers
48 views

How to choose a sample size for two group repeated measures anova design given power, alpha, and effect size?

How would I go about computing the sample size for repeated measures ANOVA design, given power, significance and the required effect size (2 groups, I know the mean difference that I want to be able ...
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1answer
50 views

How can you derive the sample size to obtain a particular type II error rate?

I often see that the sample size for a $z$-test to achieve a particular type II error rate $\beta$ (at a given significance level, $\alpha$) is: $$n = ...
2
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1answer
39 views

Simulation power analysis

How do you perform a power analysis for linear mixed model using R? I measure area of five groups of plants across time. Each group of plants grow under two treatments. I have 12 replicates for each ...
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0answers
28 views

How to calculate required sample size to detect a significant interaction between two continuous variables

I am planning to do a statistical power analysis, and I would like to know how to calculate the required sample size to detect an interaction between two continuous predictors in the presence of other ...
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0answers
8 views

How do you do a power calculation for a 2-phase prevalence survey?

I'm trying to work out what sample size I need to calculate a prevalence rate within certain confidence intervals, using a two-phase design. There is a screener test. Everyone that screens positive ...
2
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1answer
79 views

Why does one report statistical power only when results are non significant?

It is well known that a study with low statistical power has a reduced chance of detecting a true effect. But if low power also reduces the likelihood that a statistically significant result reflects ...
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4answers
2k views

Why do political polls have such large sample sizes?

When I watch the news I've noticed that the Gallup polls for things like presidential elections have [I assume random] sample sizes of well over 1,000. From what I remember from college statistics ...
11
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1answer
429 views

Is a priori power analysis essentially useless?

I attended a meeting of the Society for Personality and Social Psychology last week where I saw a talk by Uri Simonsohn with the premise that using an a priori power analysis to determine sample size ...
3
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1answer
75 views

Interpretation of power and detectable difference

I know power is the likelihood of correctly rejecting the null hypothesis, but I just want to check that my interpretation of it with the detectable difference is correct. Consider a 2 sample test ...
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0answers
17 views

Exposure in Power Calculations

When estimating power calculations for a case control study, what does exposure of controls really mean? I find lots of information how how to calculate exposure for cases when you already know ...
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0answers
17 views

Correlation among repeated measures

Can someone explain what 'correlation among repeated measures' in G*power exactly means? I believe it is the test-retest reliability, but why is it important and what are the effects on the inferred ...
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0answers
15 views

Interpretation of results of power analysis with regard to the null hypothesis

I have recently read a paper, in which the authors conducted a $2*3$ Between-within-ANOVA. The alternative hypothesis was formulated with regard to an interaction effect, which did not reach the ...
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2answers
136 views

Adjusting any power analysis with FPC?

Is it correct (perhaps at least as a good approximation) to determine sample size via a power analysis that assumes an infinite population and then to adjust the required sample size by a finite ...
2
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0answers
18 views

Increasing or decreasing sample size with higher correlation among repeated measures in G*Power

When I conduct an a priori power analysis to calculate the required sample size with G* power for a repeated measures design with between factors, the sample size increases when I insert a higher ...
3
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1answer
76 views

Everything else held constant, what is the relationship between alpha and beta?

I want the mathematical formula the statistical programs are using to compute alpha and beta. I typically use G-Power to estimate the necessary sample size for a given power (1-beta) for an a-priori ...
3
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0answers
76 views

Simulation for power analysis for higher order repeated measures ANOVA

I am attempting to find a way to perform a power analysis for a higher order repeated measures ANOVA where all factors are within-subjects (i.e., there are no between-subjects factors). I have looked ...
6
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2answers
205 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 ...
1
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1answer
102 views

A priori power analysis for generalized linear mixed-effects model

Let's say I have a data set (with a binary DV) which I used a generalized linear mixed-effects model on. I am currently trying to run a simulation-based power analysis to determine the sample size I ...
5
votes
1answer
53 views

Quantile regression power analysis

New to the site and to stats here! This may be a silly question, but I haven't been able to find a satisfactory answer on the procedure for a power analysis (or general guidelines about sample size) ...
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0answers
57 views

Sample size to compare ' change from baseline' proportions between two groups?

I have frequencies for variable with answers Yes and NO for two time points (at baseline and after treatment). The frequencies are recorded for two treatments. I would like to compare the change from ...
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0answers
18 views

Power analysis for log-gamma model

I fitted a generalized linear mixed log-gamma model in SPSS with 1 fixed factor (binary), patient ID as a random intercept and a continuous variable as (fixed) covariate. How can I calculate the ...
0
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1answer
42 views

Is there a way to measure the viability of an experiment?

Is there a way to calculate a number that evaluates the viability of an experiment? By that I mean the probability that the observed probability is just chance. I read about the $p$ value, but it ...
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0answers
21 views

How to link the SE(p) of the binomial distribution $B(n,p)$ to the SE(coefficient) in GLM?

This is a general question regarding GLM with binomial distribution. I use the following data (with $N=400$ observations) as an example (predict toxicity using treatment): ...
0
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1answer
60 views

Power of Seasonal Kendall Trend test

I was asked about doing a power analysis of a seasonal kendall trend test. I feel like that would be really difficult to do and I haven't found any documentation or software on how to do it. Is there ...
2
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0answers
58 views

Statistical power of unit root test

When studying time series, I once heard the statement that unit root test is less powerful. I hereby have two questions: What does it mean for a test to be powerful? What causes the unit root ...
8
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1answer
296 views

Two-sample comparison of proportions, sample size estimation: R vs Stata

Two-sample comparison of proportions, sample size estimation: R vs Stata I got different results for sample sizes, as follows: In R ...
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0answers
42 views

Multiparameter log-likelihood function in R and power analysis (by using simulations)

I would like to estimate power of the following problem. I am interested in comparing two groups that both follow e.g. Weibull distribution. So, group A has two parameters (...
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1answer
95 views

Sample size calculation for a single proportion Without Any Comparisons (descriptive only): How to determine the margin of error in the formula?

I want to calculate the sample size for a single proportion (the prevalence of a disease). I know the formula and all: n = (z/m)^2 * P(1-P), where z is 1.96 (for 95% confidence level) and P stands for ...
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0answers
40 views

Unstandardized effect size for two non-equal size samples for experimental design in R

I am trying to calculate the required (unstandardized) difference in between a test and control group of different sizes. I have historical data with transactions for the test group (n=272, mean ...
0
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0answers
32 views

What is the Comparison p used in a sample size calculation for a 1-proportion test in Minitab?

In order to calculate a sample size for a 1-proportion test in Minitab, one has to input the power and the comparison proportion. Could someone provide a definition for the comparison proportion ...
2
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0answers
46 views

Noncentrality parameter in two-sample-t-test (Power analysis)

I am working with R, doing a post-hoc power analysis. Can anyone explain to me why these two commands generate different outputs? I thought they should not! ...
2
votes
1answer
59 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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0answers
17 views

Find number needed for confidence level from population standard deviation

I need to know how to get the number of people needed in a survey for a 99% confidence level, within 1 person? And the only given is the population standard deviation of 15.4.
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0answers
178 views

Power Analysis for repeated measures ANOVA

I am trying to run a post-hoc power analysis on a previously published report. The intent is to calculate the average obtained power for (the equivalent of) Cohen's effects of d = .20, .50, .80) for ...
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0answers
55 views

Power Analysis and effect size

Appology if it is rather long. But i'm confused! What assumptions do i need in order to answer this question? With having two segments in the population that you wish to compare with respect to their ...