Linked Questions

2 votes
1 answer
10k views

t-test using a sample less than 30 [duplicate]

When I do the two tailed t-test using a sample less than 30, can I use this result? This is because I do also still see the t-test table which has degrees of freedom far less than 30. How further ...
  • 424
1 vote
0 answers
881 views

Unpaired t-test: large samples, unequal variances, unequal sample sizes [duplicate]

I a have two independent samples one with more than 700 observations and the second with more than 500 observations. I firstly use Cramér-von Mises test to test normality just to check how non-normal ...
1 vote
1 answer
677 views

Should I use the t-test or the Wilcoxon rank sum test given these qqplots and Shapiro-Wilk stats [duplicate]

I have 4 groups and I want to test if the pairwise difference in means are significantly different. There are 6 pairwise differences. The QQnorm plots of the 4 groups look like this: The Shapiro-...
  • 163
2 votes
0 answers
435 views

Need and best way to determine normality of data [duplicate]

I believe there are many ways to determine if data is normal or not: histogram shape QQ plot skewness kurtosis shapiro test. Which of the above is the best way to determine if data is normal and ...
  • 9,259
1 vote
1 answer
203 views

Is it valid to conduct t-tests with just 20 students? [duplicate]

By reading article I think I found a bug: Seams to me that 20 is a very small number of samples in this article assuming 20 students are taken random and the order of students is not important. I ...
0 votes
0 answers
335 views

Test statistics in IR (t-test vs Wilcoxon vs signed test) [duplicate]

I have just read this paper about different test statistics one could use in Information Retrieval (IR), discussing whether Wilcoxon, Sign-test or Student t-test would be more reliable for comparing ...
3 votes
0 answers
131 views

T-test for a small sample with unknown distribution [duplicate]

Consider a simple hypothesis test concerning the mean of a single sample. If the sample is normally distributed and the variance is known, the exact distribution of the sample mean is known ($N$~$(\...
  • 1,736
367 votes
15 answers
136k views

Is normality testing 'essentially useless'?

A former colleague once argued to me as follows: We usually apply normality tests to the results of processes that, under the null, generate random variables that are only asymptotically or ...
  • 14.1k
113 votes
7 answers
187k views

T-test for non normal when N>50?

Long ago I learnt that normal distribution was necessary to use a two sample T-test. Today a colleague told me that she learnt that for N>50 normal distribution was not necessary. Is that true? If ...
  • 2,257
76 votes
15 answers
11k views

Why would parametric statistics ever be preferred over nonparametric?

Can someone explain to me why would anyone choose a parametric over a nonparametric statistical method for hypothesis testing or regression analysis? In my mind, it's like going for rafting and ...
  • 927
57 votes
10 answers
4k views

What are some examples of anachronistic practices in statistics?

I am referring to practices that still maintain their presence, even though the problems (usually computational) they were designed to cope with have been mostly solved. For example, Yates' ...
53 votes
8 answers
36k views

When conducting a t-test why would one prefer to assume (or test for) equal variances rather than always use a Welch approximation of the df?

It seems like when the assumption of homogeneity of variance is met that the results from a Welch adjusted t-test and a standard t-test are approximately the same. Why not simply always use the Welch ...
99 votes
2 answers
14k views

How much do we know about p-hacking "in the wild"?

The phrase p-hacking (also: "data dredging", "snooping" or "fishing") refers to various kinds of statistical malpractice in which results become artificially statistically significant. There are many ...
  • 21.6k
27 votes
4 answers
17k views

Practically speaking, how do people handle ANOVA when the data doesn't quite meet assumptions?

This isn't a strictly stats question--I can read all the textbooks about ANOVA assumptions--I'm trying to figure out how actual working analysts handle data that doesn't quite meet the assumptions. I'...
  • 798
28 votes
5 answers
20k views

How robust is the independent samples t-test when the distributions of the samples are non-normal?

I've read that the t-test is "reasonably robust" when the distributions of the samples depart from normality. Of course, it's the sampling distribution of the differences that are important. I have ...

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