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help:indep_ttest [2012/08/12 18:19] (current)
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 +[[http://​www.sofastatistics.com/​userguide.php | Contents]]
  
 +[[:​help:​stats_tests | Statistical Tests Available]]
 +
 +====== Independent t-test ======
 +
 +The independent t-test is of the most well-known statistical tests (see [[http://​en.wikipedia.org/​wiki/​Student'​s_t-test | Student'​s t-test]]). ​ It is useful for looking at differences in the average between two groups when the variable being averaged is numerical and inadequately normal - for example, looking at the difference in height between countries. ​ If you are looking at paired data e.g. pre and post-diet weight, you need to use the [[:​help:​paired_ttest | paired t-test]].
 +
 +If you want to look for differences between multiple groups, the [[:​help:​anova | ANOVA (Analysis of Variance)]] is required. If your data is not normally distributed,​ N is less than 20, or there are outliers, you should probably use the [[:​help:​mann_whitney | Mann-Whitney U]].
 +
 +The p value tells you how likely the difference observed would occur if drawing from the same population (i.e. where the groups didn't really have a relationship with the value being averaged). A small p value tells you that it would be rare to observe such a difference if the group doesn'​t have a relationship with the value being averaged. From this we might reject the null hypothesis in favour of the alternative hypothesis - namely, that there is a difference according to the grouping variable.
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 +In the example below (based on false data for illustration only), we shouldn'​t be surprised the p value is very low. It is possible to see a large difference in average age and N is reasonably large for each group. In this case we could reject the hypothesis that nation has no relationship with age. 
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 +{{:​help:​indep_ttest_example.gif|}}
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 +Be aware that there is a certain sensitivity about terminology around this area.  According to a widespread convention, we shouldn'​t conclude that there is a relationship,​ only that we reject the null hypothesis (see [[http://​www.stats.gla.ac.uk/​steps/​glossary/​hypothesis_testing.html#​h0 | Hypothesis testing]]). We might go so far as to reject the null hypothesis in favour of the alternative hypothesis. ​ See [[http://​en.wikipedia.org/​wiki/​Statistical_hypothesis_testing | Statistical hypothesis testing]].
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 +[[https://​www.youtube.com/​watch?​v=-mc_pLdd6Jg&​list=UUFRr0ugWcqCfhLwJ5qBlzpQ |{{:​help:​play_button.gif|}}]] Another video is also available showing how to do an independent t-test using SOFA Statistics: [[https://​www.youtube.com/​watch?​v=-mc_pLdd6Jg&​list=UUFRr0ugWcqCfhLwJ5qBlzpQ]]
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 +[[http://​www.sofastatistics.com/​userguide.php | Contents]]
 +
 +[[:​help:​stats_tests | Statistical Tests Available]]
 +
 +[[:home | Wiki]]
help/indep_ttest.txt ยท Last modified: 2012/08/12 18:19 (external edit)