Polypompholyx

Author's details

Name: Steve Cook
Date registered: 16/04/2012
URL: http://www.polypompholyx.com

Biography

Nerd of this parish.

Latest posts

  1. Organism of the week #20 – Don’t point that thing at me — 06/03/2014
  2. Nonlinear regression — 03/03/2014
  3. Analysis of variance: ANOVA (2 way) — 24/02/2014
  4. Analysis of variance: ANOVA (1 way) — 24/02/2014
  5. Comparison of expected and observed count data: the χ² test — 17/02/2014

Most commented posts

  1. Educational RCTs — 17 comments
  2. A brief history of rubbish — 5 comments
  3. Moose objects and roles — 3 comments
  4. Organism of the week # 6 – Enigmata of the Gnetales — 2 comments
  5. Obfuscated pancakes — 2 comments

Author's posts listings

Mar 06

Organism of the week #20 – Don’t point that thing at me

Diadema setosum [CC-BY-SA-3.0 Steve Cook]

It’s amazing how informative an anus can be. Take this sea urchin. The orange pucker in the middle of the spines is its “around-the-bum”, although zoologists would insist on writing that in Greek as “periproct“. The bright orange ring-piece is characteristic of this species, and marks it out as Diadema setosum, rather than any of …

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Mar 03

Nonlinear regression

Acid phosphatase nonlinear regression model [CC-BY-SA-3.0 Steve Cook]

Nonlinear regression is used to see whether one continuous variable is correlated with another continuous variable, but in a nonlinear way, i.e. when a set of x vs. y data you plan to collect do not form a straight line, but do fall on a curve that can be modelled in some sensible way by …

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Feb 24

Analysis of variance: ANOVA (2 way)

Glucose boxplot [CC-BY-SA-3.0 Steve Cook]

The technique for a one-way ANOVA can be extended to situations where there is more than one factor, or – indeed – where there are several factors with several levels each, which may have synergistic or antagonistic effects on each other. In the models we have seen so far (linear regression, one-way ANOVA) all we …

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Feb 24

Analysis of variance: ANOVA (1 way)

Venus flytrap boxplot [CC-BY-SA-3.0 Steve Cook]

Analysis of variance is the technique to use when you might otherwise be considering a large number of pairwise F and t tests, i.e. where you want to know whether a factor with more than 2 levels is a useful predictor of a dependent variable. For example, cuckoo_eggs.csv contains data on the length of cuckoo eggs laid …

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Feb 17

Comparison of expected and observed count data: the χ² test

Maize kernel histogram [CC-BY-SA-3.0 Steve Cook]

A χ2 test is used to measure the discrepancy between the observed and expected values of count data. The dependent data must – by definition – be count data. If there are independent variables, they must be categorical. The test statistic derived from the two data sets is called χ2, and it is defined as …

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Feb 10

Correlation of data: linear regression

Cricket chirps linear model [CC-BY-SA-3.0 Steve Cook]

Linear regression is used to see whether one continuous variable is correlated with another continuous variable in a linear way, i.e. can the dependent variable y be modelled with a straight-line response to changes in the independent covariate x: Here b is the estimated slope of the best-fit line (a.k.a. gradient, often written m), a …

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Feb 03

Statistical testing

Probability distribution for two flips of a fair coin [CC-BY-SA-3.0 Steve Cook]

If you want a random yes/no answer to a question, like “who should kick-off this football match?” it’s very common to entrust the decision to the flip of a coin, on the assumption that the coin doesn’t care which side gets the advantage. But what if that trust is misplaced? What if the coin gives …

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Feb 03

Comparison of means: the t test

Dog whelks histogram [CC-BY-SA-3.0 Steve Cook]

A t test is used to compare the means of two data sets, and is calculated as follows: Where x̅, s and n are the mean, standard deviation and sample size of the two data sets. A t test can also be used to compare the mean of a single data set to a reference value, …

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Feb 03

Comparison of variances: the F test

An F test is used to compare the variances of two data sets: As it is used to compare variances, the dependent data must – by definition – be numeric. As it is used to compare two distinct sets of data, these sets represent the two levels of a factor. The test statistic we use …

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Jan 30

Organism of the week #19 – Bird-brained humans

Corvus corone [CC-BY-SA-3.0 Steve Cook]

Most people who want to attract birds to their gardens put out bird-seed, or – if they’re really adventurous – mealworms. However, on the East London dockside where I lived until recently, one of the residents strewed the quayside with livers in December last year. Whether this was a deliberate ploy to attract carrion crows for …

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