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. Bagging botanic gardens — 23/04/2014
  2. Organism of the week #20 – Don’t point that thing at me — 06/03/2014
  3. Nonlinear regression — 03/03/2014
  4. Analysis of variance: ANOVA (2 way) — 24/02/2014
  5. Analysis of variance: ANOVA (1 way) — 24/02/2014

Most commented posts

  1. Educational RCTs — 17 comments
  2. A brief history of rubbish — 5 comments
  3. Moose objects and roles — 3 comments
  4. Glockentierchen — 2 comments
  5. The Michaelis-Menten model is not applicable to most enzymes in a cell — 2 comments

Author's posts listings

Apr 23

Bagging botanic gardens

Kew gardens temperate house [CC-BY-2.0 Alex Lomas]

I’m not sure whether bagging botanical gardens is better or worse than bagging Munros, Michelin stars or the numbers off of rolling stock, but it keeps me off the streets… London (Kew) Just ten stops down the District Line from $WORK lies the Royal Botanic Gardens Kew. The gardens have three enormous glasshouses, a number of smaller …

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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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