Questions tagged [sample-size]

This tag is very ambiguous. Use it when the question is about sample size and NONE of the following are more appropriate: [small-sample], [large-data], [power-analysis], [power], [underdetermined] or [unbalanced-classes].

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93
votes
3answers
90k views

Does an unbalanced sample matter when doing logistic regression?

Okay, so I think I have a decent enough sample, taking into account the 20:1 rule of thumb: a fairly large sample (N=374) for a total of 7 candidate predictor variables. My problem is the following: ...
148
votes
16answers
39k views

Are large data sets inappropriate for hypothesis testing?

In a recent article of Amstat News, the authors (Mark van der Laan and Sherri Rose) stated that "We know that for large enough sample sizes, every study—including ones in which the null hypothesis of ...
29
votes
6answers
92k views

Sample size for logistic regression?

I want to make a logistic model from my survey data. It is a small survey of four residential colonies in which only 154 respondents were interviewed. My dependent variable is "satisfactory transition ...
85
votes
10answers
328k 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 ...
54
votes
2answers
88k views

How should one interpret the comparison of means from different sample sizes?

Take the case of book ratings on a website. Book A is rated by 10,000 people with an average rating of 4.25 and the variance $\sigma = 0.5$. Similarly Book B is rated by 100 people and has a rating of ...
44
votes
4answers
92k views

What references should be cited to support using 30 as a large enough sample size?

I have read/heard many times that the sample size of at least 30 units is considered as "large sample" (normality assumptions of means usually approximately holds due to the CLT, ...). Therefore, in ...
76
votes
7answers
226k views

Rules of thumb for minimum sample size for multiple regression

Within the context of a research proposal in the social sciences, I was asked the following question: I have always gone by 100 + m (where m is the number of predictors) when determining minimum ...
21
votes
1answer
3k views

Is there a statistical test to compare two samples of size 1 and 3?

For an ecology project, my lab group added vinegar to 4 tanks containing equal volumes of pond water, 1 control with no elodea (an aquatic plant) and 3 treatments with the same amount of elodea in ...
30
votes
7answers
25k views

What is the minimum recommended number of groups for a random effects factor?

I'm using a mixed model in R (lme4) to analyze some repeated measures data. I have a response variable (fiber content of feces) ...
30
votes
1answer
53k views

Why is the sampling distribution of variance a chi-squared distribution?

The statement The sampling distribution of the sample variance is a chi-squared distribution with degree of freedom equals to $n-1$, where $n$ is the sample size (given that the random variable of ...
17
votes
2answers
8k views

Effective Sample Size for posterior inference from MCMC sampling

When obtaining MCMC samples to make inference on a particular parameter, what are good guides for the minimum number of effective samples that one should aim for? And, does this advice change as the ...
20
votes
4answers
17k views

Calculating required sample size, precision of variance estimate?

Background I have a variable with an unknown distribution. I have 500 samples, but I would like demonstrate the precision with which I can calculate variance, e.g. to argue that a sample size of 500 ...
19
votes
3answers
2k views

Example of distribution where large sample size is necessary for central limit theorem

Some books state a sample size of size 30 or higher is necessary for the central limit theorem to give a good approximation for $\bar{X}$. I know this isn't enough for all distributions. I wish ...
47
votes
5answers
8k views

What can we say about population mean from a sample size of 1?

I am wondering what we can say, if anything, about the population mean, $\mu$ when all I have is one measurement, $y_1$ (sample size of 1). Obviously, we'd love to have more measurements, but we can'...
13
votes
2answers
2k views

Minimum sample size per cluster in a random effect model

Is there a rational for the number of observations per cluster in a random effect model? I have a sample size of 1,500 with 700 clusters modeled as exchangeable random effect. I have the option to ...
37
votes
1answer
48k views

Determining sample size necessary for bootstrap method / Proposed Method

I know this is a rather hot topic where no one really can give a simple answer for. Nevertheless I am wondering if the following approach couldn’t be useful. The bootstrap method is only useful if ...
15
votes
2answers
7k views

How to choose significance level for a large data set?

I am working with a data set having N around 200,000. In regressions, I am seeing very small significance values << 0.001 associated with very small effect sizes, e.g. r=0.028. What I'd like to ...
28
votes
4answers
35k views

Minimum sample size for PCA or FA when the main goal is to estimate only few components?

If I have a dataset with $n$ observations and $p$ variables (dimensions), and generally $n$ is small ($n=12-16$), and $p$ may range from small ($p = 4-10$) to perhaps much larger ($p= 30-50$). I ...
16
votes
8answers
69k views

Minimum sample size for unpaired t-test

Is there a "rule" to determine the minimum sample size required for a t-test to be valid? For example, a comparison needs to be performed between the means of 2 populations. There are 7 data points ...
10
votes
2answers
2k views

Does it make sense to compute confidence intervals and to test hypotheses when data from whole population is available?

Does it make sense to compute confidence intervals and to test hypotheses when data from the whole population is available? In my opinion, the answer is no, since we can accurately compute the true ...
9
votes
4answers
27k views

Minimum number of observations for logistic regression?

I'm running a binary logistic regressions with 3 numerical variables. I'm suppressing the intercept in my models as the probability should be zero if all input variables are zero. What's minimal ...
6
votes
2answers
883 views

How to report general precision in estimating correlations within a context of justifying sample size?

I have a study with 130 participants where most of my analyses involve examining the relative size of correlations between various psychological variables. I'm trying to comply with recommendations ...
2
votes
1answer
8k views

Mann-Whitney U test and K-S test with unequal sample sizes

I want to compare two distributions, to see if they are significantly different. They represent task time completions (so they range from 1 to around 1000 seconds) in two different months. They are ...
33
votes
5answers
9k 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 ...
15
votes
3answers
24k views

When is Fisher's z-transform appropriate?

I want to test a sample correlation $r$ for significance, using p-values, that is $H_0: \rho = 0, \; H_1: \rho \neq 0.$ I have understood that I can use Fisher's z-transform to calculate this by $...
1
vote
1answer
339 views

Experimental Design on Testing Proportions

If you wanted to design an experiment to test the proportion of successes under two different strategies, and you had a fixed number of trials -- are you better off balancing the number of trials? (e....
4
votes
2answers
13k views

Sample size for unequal groups in logistic regression

I am conducting a study, in which I will be using Logistic Regression for analysis. I will be collecting data in an education setting (about cancer prevention), then I will follow subjects to collect ...
13
votes
1answer
4k views

Power analysis from Bayesian point of view [duplicate]

I was reading an article on a power analysis and sample size calculations for a class of models that known as joint longitudinal-survival models. The paper derives a closed form formula for power and ...
17
votes
2answers
29k views

Mann-Whitney U test with unequal sample sizes

I have two unequal groups (94 and 52) and want to run a Mann-Whitney U-test to see whether their scores on a measured variable differ. I see that it is OK to do with Kruskall-Wallis, does the same ...
9
votes
3answers
10k views

How to correct for small p-value due to very large sample size

I am running into a problem where an independent variable, which should have no predictive power on the dependent variable based on domain knowledge, comes out with very small p-value because the ...
8
votes
2answers
5k views

Does a big difference in sample sizes together with a difference in variances matter for a t-test (or permutation test)?

There is a very confusing question in my mind. I have data, and would like to compare numeric scores between men and women. There is a big difference in those two groups: the number of men is 34, ...
8
votes
1answer
2k views

How to calculate a sample size for validating correct/incorrectness of records in a data table?

I have read through existing answers on CrossValidated (plus elsewhere online) and can't find what I'm looking for, but do please point me to existing sources if I've missed them. Let's say I have a ...
15
votes
3answers
51k views

How do you decide the sample size when polling a large population?

Australia is currently having an election and understandably the media reports new political poll results daily. In a country of 22 million what percentage of the population would need to be sampled ...
11
votes
3answers
13k views

Power analysis for binomial data when the null hypothesis is that $p = 0$

I'd like to do a power analysis for a single sample from binomial data, with $H_0: p = 0$, vs. $H_1: p = 0.001$, where $p$ is the proportion of successes in the population. If $0 < p <1$, I ...
5
votes
2answers
9k views

Independent samples t-test with unequal sample sizes

I have two independent groups: from one I have drawn a sample of size less than 30 due to its small population, while the second group has sample size 100. Is it appropriate to use an independent ...
11
votes
1answer
2k views

Is bootstrapping appropriate for this continuous data?

I'm a complete newbie :) I'm doing a study with a sample size of 10,000 from a population of about 745,000. Each sample represents a "percentage similarity". The great majority of the samples are ...
24
votes
3answers
15k views

Sanity check: how low can a p-value go?

I am using a ranksum test to compare the median of two samples ($n=120000$) and have found that they are significantly different with: p = 1.12E-207. Should I be ...
11
votes
4answers
7k views

Sample size calculation for univariate logistic regression

How does one calculate the sample size needed for a study in which a cohort of subjects will have a single continuous variable measured at the time of a surgery and then two years later they will be ...
5
votes
1answer
56k views

What is a reasonable sample size for correlation analysis for both overall and sub-group analyses?

I am running Pearson's correlation on an overall sample of 400 respondents. When I isolate male and female responses, my sample becomes 220 male responses and 180 female responses. If I further ...
2
votes
1answer
6k views

Sample size for categorical data

I have a population of phone calls - 200,000. There are different reasons for each call, but lets assume the number of reasons is known. i.e. 7 different call reasons: 1) Check on order 2) Cancel ...
6
votes
2answers
2k views

Sample size for a variable number of answers

I have N different possible results (ex. red, yellow, or blue). I go to a population of infinite size and ask a bunch of people a question the answer to which is one of the N options.... How large ...
3
votes
1answer
4k views

Likert item as independent variable for ANOVA?

I am trying to analyze data based on a sample size of around 50 and would appreciate advice on which statistical test would be most appropriate. Participants, on a single questionnaire, were asked to ...
12
votes
4answers
10k views

How large should a sample be for a given estimation technique and parameters?

Is there a rule-of thumb or even any way at all to tell how large a sample should be in order to estimate a model with a given number of parameters? So, for example, if I want to estimate a least-...
3
votes
1answer
9k views

Sample size calculation for ROC/AUC analysis

As a background, I am not familiar with stats except on a basic level. I have been tasked with doing some analysis that is out of my comfort zone. I am trying to figure out how to compute necessary ...
3
votes
1answer
6k views

Mann Whitney or two tailed t-test

I am new to statistics and am in need of some help. I am running a study where I am providing two groups with surveys to test for their perceptions using a Likert Scale. One group I'm testing prior to ...
3
votes
2answers
2k 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
votes
2answers
1k views

ANOVA with huge dataset - use only the mean for each condition?

I need advice about how to carry out an ANOVA. I studied some theory of ANOVA, but apparently it is not enough. Basically I collected around 300 reaction times for 12 subjects in my experiment. For ...
6
votes
2answers
2k views

How to adjust average rating for sample size on rating systems with more than two categories?

After reading How Not To Sort By Average Rating which deals with confidence interval for a Bernoulli parameter how would you extend it to more than two levels? For example: Items are scored between 1 ...
14
votes
2answers
52k 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 ...
41
votes
10answers
10k views

Why is 600 out of 1000 more convincing than 6 out of 10?

Look at this excerpt from "The study skills handbook", Palgrave, 2012, by Stella Cottrell, page 155: Percentages Notice when percentages are given. Suppose, instead, the statement above read: ...