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Distractions and amusements, with a sandwich and coffee.

Feel the vibe, feel the terror, feel the pain
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• Mad about you, orchestrally.
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On March 14th celebrate `\pi` Day. Hug `\pi`—find a way to do it.

For those who favour `\tau=2\pi` will have to postpone celebrations until July 26th. That's what you get for thinking that `\pi` is wrong. I sympathize with this position and have `\tau` day art too!

If you're not into details, you may opt to party on July 22nd, which is `\pi` approximation day (`\pi` ≈ 22/7). It's 20% more accurate that the official `\pi` day!

Finally, if you believe that `\pi = 3`, you should read why `\pi` is not equal to 3.

How many ages hence

Shall this our lofty scene be acted over

In states unborn and accents yet unknown!

—Willian Shakespeare (Julius Caesar, Act III, Scene 1)

Welcome to this year's celebration of `\pi` and mathematics.

The theme this year is typographical and pure—in contrast to last year's extremely elaborate computational art. This year is also the first time I have made a special kids' edition!

This year's poem is by Viorica Hrincu and it is about complications.

This year's `\pi` day song is Tshinanu by Kashtin.

If you enjoy art based on type, explore my other typographical works.

Several teachers have reached out to me in the past and asked for art to hang in their classrooms. So I thought what better way to get kids excited and talking (and reading) about math than with an explosion of colors and phat fonts.

The international kids' poster uses the following languages: Afrikaans, Albanian, Azerbaijani, Basque, Catalan, Cebuano, Corsican, Croatian, Danish, Dothraki, Dutch, English, Esperanto, Estonian, Finnish, French, Frisian, Galician, German, Gongduk, Haitian/Creole, Hausa, Hawaiian, Huastec/Mayan, Icelandic, Indonesian, Irish, Italian, Khasi, Ladan, Latin, Latvian, Lithuanian, Luxembourgish, Malagasy, Maltese, Maori, Nao, Norwegian, Polish, Portuguese, Puyuma, Ro, Samoan, Scots/Gaelic, Sesotho, Shona, Slovak, Slovenian, Solon, Solresol, Somali, Spanish, Swahili, Swedish, Tagalog, Tenerife, Tetun, Turkish, Welsh, Xhosa, Yoruba, Zenaga, and Zulu.

Some of these languages are fictional or engineered—I'll leave it to you to find them.

The posters read out the digits of `\pi` in a variety of languages. For this version of the art, I've selected language digit words that have no diacritical marks—to fit the letters more tightly.

Adults shouldn't feel left out—they're free to enjoy the `\pi` Day kids' editions (I know I want to).

However, for the more discerning typographer in you (if Granby Elephant is too bloated for your eyes), I have prepared something slimmer.

These posters spell out the digits of `\pi` in a variety of languages and alphabets. I'll leave you to work out the rule for the red highlights.

The following languages are used: Afrikaans, Ainu, Albanian, Amharic, Arabic, Armenian, Avestan, Azerbaijani, Balinese, Banjar, Bengali, Biblical Arameic, Buginese, Bulgarian, Bumthang, Catalan, Cherokee, Chinese, Cia-cia, Coptic, Czech, Danish, Dothraki, English, Esperanto, Etruscan, Frisian, Galician, Gaulish, Georgian, Georgian Old, Gothic, Greek Old, Gujarati, Hebrew, Hindi, Hopi, Hungarian, Igbo, Inuktitut, Irish, Japanese Sino, Javanese, Kannada, Kazakh, Khmer, Klallam, Korean, Korean Sino, Kyrgyz, Ladan, Ladino, Lao, Latvian, Macedonian, Malagasy, Malay, Malayalam, Mandaic, Mongolian Classical, Myanmar, Norse, Norwegian, Old English, Old Turkic, Pashto, Persian, Phoenician, Proto Germanic, Punjabi, Ro, Romanian, Samaritan, Samoan, Sanskrit, Sesotho, Sindhi, Sinhala, Slavonic, Slovak, Slovenian, Spanish, Sundanese, Swahili, Swedish, Sylheti, Syriac, Tajik, Telugu, Tetun, Thai, Thompson, Turkish, Ugaritic, Urdu, Welsh, Wyandot, Xhosa, Yi, Yiddish, and Yonaguni.

The typeface is Helvetica Neue or Noto Sans.

In another version of the poster, instead of using words to spell out the digits, I use digit glyphs from various alphabets. Included in this version are also any single-glyph words for the digit.

The digit glyph poster uses Adlam, Ainu, Arabic, Balinese, Bengali, Brahmi, Burmese, Chakma, Cham, Chinese, Coptic, Ethiopic, Gujaranti, Gujarati, Gurmukhi, Hebrew, Japanese Sino, Javanese, Kannada, Kayahli, Kharoshthi, Khmer, Klingon, Korean, Koreansino, Lao, Lepcha, Limbu, Malayalam, Meetei Mayek, Mongolian, Nko, Odia, Olchiki, Oriya, Osmanya, Punjabi, Roman, Rumi, Sanskrit, Saurashtra, Shan, Sinhala, Sinhala Archaic, Sumerian, Sundanese, Tamil, Telugu, Thai, Tibetan, Vai, Yi, and Yonaguni.

*You can look back there to explain things,
but the explanation disappears.
You'll never find it there.
Things are not explained by the past.
They're explained by what happens now.
—Alan Watts*

A Markov chain is a probabilistic model that is used to model how a system changes over time as a series of transitions between states. Each transition is assigned a probability that defines the chance of the system changing from one state to another.

Together with the states, these transitions probabilities define a stochastic model with the Markov property: transition probabilities only depend on the current stateâ€”the future is independent of the past if the present is known.

Once the transition probabilities are defined in matrix form, it is easy to predict the distribution of future states of the system. We cover concepts of aperiodicity, irreducibility, limiting and stationary distributions and absorption.

This column is the first part of a series and pairs particularly well with Alan Watts and Blond:ish.

Grewal, J., Krzywinski, M. & Altman, N. (2019) Points of significance: Markov Chains. *Nature Methods* **16**:663–664.

*Places to go and nobody to see.*

Exquisitely detailed maps of places on the Moon, comets and asteroids in the Solar System and stars, deep-sky objects and exoplanets in the northern and southern sky. All maps are zoomable.

Quantile regression explores the effect of one or more predictors on quantiles of the response. It can answer questions such as "What is the weight of 90% of individuals of a given height?"

Unlike in traditional mean regression methods, no assumptions about the distribution of the response are required, which makes it practical, robust and amenable to skewed distributions.

Quantile regression is also very useful when extremes are interesting or when the response variance varies with the predictors.

Das, K., Krzywinski, M. & Altman, N. (2019) Points of significance: Quantile regression. *Nature Methods* **16**:451–452.

Altman, N. & Krzywinski, M. (2015) Points of significance: Simple linear regression. *Nature Methods* **12**:999–1000.

Outliers can degrade the fit of linear regression models when the estimation is performed using the ordinary least squares. The impact of outliers can be mitigated with methods that provide robust inference and greater reliability in the presence of anomalous values.

We discuss MM-estimation and show how it can be used to keep your fitting sane and reliable.

Greco, L., Luta, G., Krzywinski, M. & Altman, N. (2019) Points of significance: Analyzing outliers: Robust methods to the rescue. *Nature Methods* **16**:275–276.

Altman, N. & Krzywinski, M. (2016) Points of significance: Analyzing outliers: Influential or nuisance. Nature Methods 13:281–282.

Two-level factorial experiments, in which all combinations of multiple factor levels are used, efficiently estimate factor effects and detect interactionsâ€”desirable statistical qualities that can provide deep insight into a system.

They offer two benefits over the widely used one-factor-at-a-time (OFAT) experiments: efficiency and ability to detect interactions.

Since the number of factor combinations can quickly increase, one approach is to model only some of the factorial effects using empirically-validated assumptions of effect sparsity and effect hierarchy. Effect sparsity tells us that in factorial experiments most of the factorial terms are likely to be unimportant. Effect hierarchy tells us that low-order terms (e.g. main effects) tend to be larger than higher-order terms (e.g. two-factor or three-factor interactions).

Smucker, B., Krzywinski, M. & Altman, N. (2019) Points of significance: Two-level factorial experiments *Nature Methods* **16**:211–212.

Krzywinski, M. & Altman, N. (2014) Points of significance: Designing comparative experiments.. Nature Methods 11:597–598.

Digits, internationally

Celebrate `\pi` Day (March 14th) and set out on an exploration explore accents unknown (to you)!

This year is purely typographical, with something for everyone. Hundreds of digits and hundreds of languages.

A special kids' edition merges math with color and fat fonts.

Check out art from previous years: 2013 `\pi` Day and 2014 `\pi` Day, 2015 `\pi` Day, 2016 `\pi` Day, 2017 `\pi` Day and 2018 `\pi` Day.